1999 Mazda B2500 (Ford Ranger) revs own engine

I've got an odd problem with my 1999 Mazda B2500 (Ford Ranger under the
hood), I'll post as much information about it as I can and see if it
makes sense in the end.

About two months ago, I had the following work done on the truck, in
the following order, all within about a week of each other:

1=2E General oil change/filter change/rotate tires at Jiffy Lube. Check
all fluids, top off anything that needs it.
2=2E Regular tune-up at the dealership (Carmax), including replacing the
spark plugs (four cylinder engine), check all filters/belts/hoses, etc.
They didn't bother checking the battery (you'll see why I mention
that later). They "tweaked" (to use their words) the way the truck
was idling in the computer settings.
3=2E Two or three days later, the truck won't start. First person to
look at it and jump it says my battery is dead. I drive to an auto
parts store down the block, they check it and say it's not the
battery but the alternator. I drive it to my mechanic, he pulls out
the alternator, tests it, says it's charging fine, then tests my
battery and says that's the real problem. He puts my original
alternator in and a new battery.

After all this, things run fine for a couple of days. Then the
problems start.

They can be best described as the engine running too high. When I turn
it on first, it idles at about 1,200rpms for a few seconds, then drops
to around 750rpms (what it should normally idle at). When I get on the
road and start going, the engine problems come up. First, when put the
clutch all the way in to shift up (or coast to a stop), the engine will
either maintain its speed when I put the clutch in or actually increase
speed (usually gradually over a few seconds, not a jump all at once).
It will go well over 3,000rpms before I either turn the engine off or
put the stick up into 5th gear and ease off the clutch a hair at a time
to try to choke it off (depending on if I'm stopped dead in traffic
or up-shifting on the road). I normally shift between 2,000 and
3,000rpms, depending on whether I need a lot of horsepower (like
getting on a highway) or if it's cold outside and I'm trying not to
push her too hard until she warms up.

This happens intermittently and randomly, both when the engine is cold
and when it's hot from driving for hours non-stop. As I said, it
happens when I push the clutch in to either shift or to come to a stop.
It will occasionally do this also when stopped dead, not moving (just
sitting idle).

I've also noticed that when driving down the road, the engine will
occasionally maintain speed without giving it any gas. My clutch is
rather old, so I can feel a jerk if I take my foot off the gas for a
few seconds and then put it back in (even when I ease it in gently, you
can still feel it "grab" when it first gets started). The vehicle
does this occasionally on its own - I take my foot off the gas to
cruise a little and after a few seconds I notice I'm maintaining
speed, without touching the gas, and the cab will jerk a bit as if I
had gunned the engine.

All of these things have occurred, obvious, when I wasn't touching
the gas pedal. When I say it revs on its own, I mean it literally revs
on its own. I've gotten interesting (dirty) looks from people
crossing in cross-walks in front of me lately (to which they get the
sheepish "It's her fault, not mine!" look).

I've been to three mechanics a total of four times, and so far I've
been told there is nothing they can do since when they road test her,
she doesn't do anything like I've described. I thought maybe it
was my fault, even though I'm not driving her any differently.
I've been very mindful of how I'm running her the last few days,
and the problems are still there, whether I beat her hard down the road
or treat her with kid-gloves.

As I said, it happens equally often (and randomly) when the engine is
hot and cold, when it's been driven all day or left to sit all night
and started in the morning cold. The only way to stop her is to either
put it up into 5th gear (if I'm not there already) and ease the
clutch off to choke her down a bit, cut the engine, or wait until she
chokes herself out.

The only other odd thing she does (and this is something that's
happened since I bought her a couple years ago from Carmax) is a valve
on the driver's side under the hood (part of the exhaust system)
makes a clank-clank-clank noise. The valve has been replaced and I was
told that it's just a noisy valve and there is no problem with it.
It has gotten much noisier since this all started, but I haven't had
any idiot lights go off (like the last time the valve went bad, when my
dash lit up like a Christmas tree). I've noticed that when the
clank-clank-clank starts, the engine idles a little rough. I'm
hoping (honestly) that this is related to that valve, and it will go
bad again soon and set off my idiot lights, since without idiot lights
nobody can/will fix her.

At this point, I've ruined the good relationships I've had with the
only decent mechanics in my area (that I know about), all of them
thinking I'm crazy since it won't do it when they drive her. I'm
getting tired of hearing "Are you sure it's doing this?," or the
more popular phrase, "Could it be operator error?" So far, they
claim to have done the following:

Check all the hoses
Check all the valves
Check that all the fluids are topped off
Check the sensors
Check the cable on the cruise control (plenty of slack, not hanging)
Connect to a computer to dump codes (there are none)
Road test it (to the tune of about =BD a tank of gas)
Let it set idle all morning in the garage
Hook a mobile computer up to monitor the computer while driving

As I said, they can't find a problem, though it's getting worse.

Any suggestions? I'd like to get some ideas of what it COULD be
before I drive it to the Ford/Mazda dealership locally to have them
take a look at it (my last desperate hope in having this fixed).

Anything you need me to clarify, let me know.

The first thing that comes to my mind would be an IAC that intermittently
sticks full open. As we drive, the PCM will open the IAC all the way so that
it can use it for "dashpot" effect if the throttle is closed suddenly. While
some state that they are satified with the results of cleaning these valves,
I refuse to risk the reliability of my repair on cleaning and opt for
replacement.

HTH

Could it be a sticking throttle cable? I don't know if that is something they
checked. Mine certainly behaved exactly that way. When it 'revs' itself, try
pulling up on the gas pedal with your hand or foot. If it revs down when you
do that, it is almost certainly related to your throttle cable.