I've searched and read for the past two hours and have found nothing like this. Idle is normal at around 650 in neutral (auto tranny) and car runs fine for the most part. While cruising at lower speeds (30-45mph) and just maintaining speed, I get a surge (like its hunting). Throttle is steady, but the rpms increase and decrease causing the car to lurch. If I put it in neutral and rev it, the rpms bounce a couple times on the way back down to a smooth idle. It doesn't hunt at idle but it does lurch away from a complete stop intermittently. All problems are when warm and are not apparent when cold. Things I have installed to make the car run better:

MSD 6A, MSD Coil, Taylor wires and new plugs, cap and rotor
New fuel pump and filter (in tank and in-line)
Corrected a vacuum leak off the intake.

This just started today after I soldered all the wires for the MSD 6A (had the wiring kit with the spade connectors). It was running GREAT and I took it up to about 100mph to check for an old vibration problem I attributed to an intermittent miss at high rpms. The vibs are gone, but now this! Why can't it ever be DONE!?! It's fun, but it's also Christmas time and my family misses me for being in the garage too much! I built a bench and table out there to attract them, but "they didn't come!"

Guess I should tell you its a 91 L98 with 53,500 miles and stock with the exception of the ignition system.

Thanks in advance for any free advice. As I read in an earlier post, "At least I know I'm gettin my money's worth!"

 

Common causes of a hunting or stumbling idle include:

Dirty or non-functional EGR valve
Dirty or non-functional IAC motor
Bad spark plugs
Bad spark plug wires
Dirty distributor cap
Bad O2 sensor
Clogged or plugged up catalytic converter
Clogged or plugged up fuel filter
Dying fuel pump
Other various electrical issues

I'm sure there are more, and I know you changed some of these, but I am posting all I can think of for the next guy who comes along.