[Shotimes] Engine speed oscillation

[David Carmean]

I have a '90 with a lightened flywheel and an IPT Stage II (I think) clutch, 
with a very odd, highly intermittent, engine problem.

Just after I bought the car about 1.5 years ago, I had the lightened flywheel 
installed (8 lbs .. SHO Shop?  I forget whose; Not the Fidenza), and I began to 
notice that every once in a while when I'd first start the car, the idle would 
wander up and down about 3 times/second for a few seconds, and then settle down.  
That behavior had mostly gone away until recently;  Now it's happened about 
five times at-speed on the freeway/city streets.

When this happens under power, the car begins to buck violently.  The rate of 
the oscillation seems to be completely independent of engine RPM.  So far, 
once I've pulled over and shut the engine off for a couple of minutes, it'll be fine 
again for a few weeks.

The Check Engine light does *not* illuminate.  What I noticed when it happened to me 
in a parking lot earlier today was that after about four re-starts to get it to 
settle down, the tach needle did not move for several seconds, even though the 
engine was running fine.    I'm not sure, but the other instrument panel lights 
may have also been strangely dark.

Nick ran the codes for me a few weeks ago, a couple of weeks after this first happened 
at-speed, and there were no stored codes.  I guess I should go buy a code reader...

Any ideas?  Any ideas where I can get a smog manual for this car?

Thanks.