87 TBI Suburban bog / acceleration problem

I need some help with a problem that despite my best efforts persists.

The vehicle: 1987 ½ ton 4wd Suburban with 350 TBI, AT, 67K original
miles, all stock

The problem: on starting from a dead stop, there is an intermittent
severe bog until mid RPMs then acceleration is as normal or near
normal. Sometimes this bog is only on starting; sometimes it is at
the low end of each gear. When the problem is occurring, acceleration
during low RPMs is the same for full pedal, or light pedal. As I
said, the problem is intermittent. When things are working,
acceleration is great throughout and smooth and proportional to pedal
travel. The problem seems to be more prevalent when hot, but does
happen also when dead cold. No error codes are stored. MPG is not
noticeably affected (bad as it ever was). The problem is not
noticeable on the freeway. It seems to happen only on starting from a
stop, and more often after sitting idling for a significant time (more
than a minute or so).

What I have done: Changed 02, TPS, MAP, TBI gaskets, plugs, wires,
cap, rotor, fuel filter. Checked timing (0 deg with wire
disconnected), fuel pressure (13 psi), all hoses checked for vacuum
leaks. EGR operation checked (by shop). Injector cleaner run though.
Changed gas stations over last 5 tanks. I have tried disconnecting
the 02 sensor but the symptoms are the same (I do get an error code
then though). Same for the EGR system.

Other: I am not sure how the timing is supposed to work on this, but
here is how it does work. With the test wire connected, timing
appears to be advanced about 10 degrees at idle. When the throttle is
bumped, the timing goes back to zero and climbs with RPM. With the
test wire disconnected, the timing is at zero but the idle hunts a bit
(idle is steady with the test wire connected).

Thanks for any help you can provide. At this point, I would dump the
TBI and go carb, but the under the hoods in NV eliminate that option.

Disclaimer: I know some will note the "money I saved" in replacing
all the sensors etc over taking it to a shop. I did take it to one
shop here where they declared the fuel filter to be the culprit. This
was changed by the shop and victory was declared. Of course, when I
insisted on a test drive to verify the problem was solved, the problem
remained. Their next suggestion was to change the harmonic balancer
and the timing belt (timing belt on a 350?). I considered continuing
to dump my wallet, but instead opted to pay them the $170 I owed them
for the filter change and left. Since then, I have heard nothing but
horror stories about mechanics here in Vegas. I was lucky to leave
having spent that little. Many of my office mates have spent
thousands with no results. I simply don't have the time or money to
visit several shops on the slim chance of finding an honest mechanic
here. So, I will be fixing this myself….unless there is a Vegas
mechanic out there willing to let me pay only for service that
actually fixes the problem.

Regards,
Mike