I have a 1986 Camaro IROC Z that has recently developed a unique problem. When the car is in OD or drive before it's warmed up it has a rough surging idle where it goes from 600 down to 200 and back. Sometimes it even wants to stall out. It seems to do this more on cool nights than during the day. When I put it in Park or Neutral the idle smooths out. I replaced my IAC valve about six months ago, I readjusted my TPS at the same time, I replaced my EGR a little over a year ago and I did plugs and wires a little over a year ago. Any idea what it could be? A lot of past threads I read people have the opposite problem with a high rough idle in P/N and a smooth idle in OD/D. As where I have a low rough idle in OD/D.

You replaced the IAC valve and the egr. But did you clean out the passages? The passages in the intake manifold for the egr gets clogged. The passages in the TB for the iac gets clogged also. They must be cleaned. It is amazing how much carbon can get into those passages.

I did. I sprayed a **** ton of breakcleaner everywhere in the IAC. Before we did the EGR we did the Seafoam throttle body treatment too.

I'm sorry, but spraying with cleaner may not be enough. I had to take a hanger wire to scrape out the deposits. They are encrusted inside the passages. I also used a brush made to clean engine passages.