Idle Drops Immediately After Start, Then Fine

I have a '93 Tbird 5.0L that has, for as long as I've owned it, had a little quirk that irritates me.

When you start the car, it cranks, fires, revs up, then immediately drops off to an almost-stall, then catches itself and is fine for the duration of the time that you're driving. Idle quality is great, car seems to run very well.

This wouldn't be annoying except when you have the A/C on, the added drag of the compressor will sometimes cause the car to stall right after it fires. Remembering to turn the HVAC to a non-A/C mode (Off or Vent, etc.) will solve that (or pressing the throttle slightly right after it fires) but it's irritating none-the-less.

I thought it may have been a hard-to-trace vacuum leak but then pulled the engine for a little work and the problem persisted after reassembly and a thorough check-over of the vacuum system. I also considered that it might be an IAC issue but it never has a problem with idle quality other than immediately after startup.

One thing: I got this engine with out the Thermactor system. It's gone and the ports on the heads are plugged with bolts in the rear. The exhaust also came without cats.

Anyone have thoughts on this?

Thanks.

Thanks for the reply. I'll pull the IAC this weekend and clean it up or replace it.

Thanks for the reminder. I have read that post several times in the past as recreational reading but I didn't connect it with my current problem for the simple reason that the car under all other conditions seems to control its idle perfectly.

I start it, it fires without excessive cranking, the revs spike up as expected, then the immediately fall off to a near-stall but it very (and I mean very) quickly catches itself and hits the idle speed that it ought to. I just never connected my problem to the idle surge problem because aside from that one time after startup, it doesn't have issues.

It's a great resource to have, thanks for taking all of the time to generate and maintain it.