Please describe the 'SES flutters' a little better. On startup, while driving or what?
I'll pass this along just in case. I had a problem with my 86 L98 after buying it. When the weather started getting cold it would start fine, a few blocks from home it would start to buck and the SES light would come on. Had a code from what I remember that it couldn't talk to ECM, but that all seemed to check out and no other codes. When it bucked, get it home and let it sit about a 1/2 hour and it drove just fine.
One day on a fluke, I tapped the IAC with a plastic faced hammer and it cleared up. It's behaved itself till recently. Lately if you start the car when it's cold out the SES light would come on and it would hunt for idle for a couple of minutes. Once the car started warming up the SES light went out and it ran fine. Tapping it again seemed to clear it up.
Yesterday I had my mechanic do some work and while doing that changed out the IAC valve. I've had a chance to drive the car plus did 3 cold engine starts and no more SES light.
Don't know if it's your problem or not, but replacing the IAC cleared up my issue far as I can tell.
PS In my case, the battery tested ok, the alternator was rebuilt and a new charging wire on it. Additionally I'd already cleaned up all the grounds. When the SES came on it did not set any codes so it was kind of a 'seat of your pants' diagnosis and seems I made the right decision.
