>
> The first thing that comes to mind is the tendency of Miatas to eat
> spark plug wires. If yours haven't been replaced in the last 30K miles
> or so, you might want to throw a set at it and see if that makes the
> problem go away.
>

It has, as best I can tell, the original 1997 plug wires. But a bad
plug wire should cause a loss in power, right? I never feel even a
slight hiccup ......

> On line, I found this. The assumption is that a P030X code might be a
> number where X is 1 to 4 if a specific cylinder is affected:
>

According to my scan tool, it's a P0300 code. "Random/multiple"

> "Random misfires that jump around from one cylinder to another (read:
> P030x codes) also will set a P0300 code. The underlying cause is often a
> lean fuel condition, which may be due to a vacuum leak in the intake
> manifold or unmetered air getting past the airflow sensor, or an EGR
> valve that is stuck open."
>

I tested EGR last night, the valve holds vacuum and nearly kills the
engine when I start to open it manually at idle, indicating that the
passages are clear..and that the valve shuts correctly.
I will check for intake leaks tomorrow with some carb cleaner but I
would think that a leak there would cause a misfire..which I just
never seem to feel. And the fact that it only happens at freeway
speeds where manifold vacuum is going to be lower than at a low-speed
cruise/idle condition seems to rule the intake manifold out...at least
a little bit. A vacuum leak, if I have one, at cruise is going to have
less effect on the overall mixture than that same leak at idle, at
least that's how it seems to make sense to me, but I'm not an
expert....


Chris