HISTORY
For the past couple of years, I had noticed that idle would drift up momentarily right after starting or at stoplights. I assumed that my big foot was tagging the gas pedal when I had my foot on the brake. The timing of the idle going back to normal seemed to coincide with my noticing it and shifting my foot.

IAC and throttle body were cleaned last year. No change.

Last fall, I replaced the head gasket and FI water temperature sensor. I carefully cleaned the flame trap system and other vacuum passages while I had it apart. No change in idle at that time (but the water leak stopped).

Then after a few thousand miles, my daughter began to complain that the car would rev way up while sitting warm at stoplights. "Dad, the car is roaring at me!"

I purchased a new IAC and took it down to college and put it on. Since then, the car does not roar, but it surges on about a 5 second cycle, especially when cold. It slows almost to a stall and drops to three cylinders, then the engine shudders and revs up to around 1500 RPM, then drops back down again.

When fully warm, at some stoplights it idles almost normally, at others it seems to drop a cylinder and idle very slow.

I checked the TPS. It clicks and switches between 0 and infinite ohms as the throttle moves off the stop.

It set two Check Engine codes for "mixture control" right after I replaced the IAC. I cleared them and they have not recurred, but I have not driven it a lot.

I don't see any obvious vacuum leaks, but I have not gotten out the propane bottle.

One last note: The 5th fuel injector has been unplugged on this car since it was nearly new. (251k miles now)

Suggestions are greatly appreciated!!