Hi there. I have a 1988 firebird formula, manual 5 speed tranny and a 305 h.o.
TBI injected engine. When it starts from cold, the idle will go to 2500 rpm and stay that way for quite a while(like choke sticking). Then will run ok but has a rough idle when warm and at a stop...it won't really quit but seems to not run on all 8 cylinders and rpm's drops below 450 then oscilates to 900 then back down again...engine will then hesitate when i give it gas. The engine temperature is descent but this only happens when engine is warmed up. I did change the IAC valve and I also changed the throttle body gasket because I detected a air leak(using the old carb cleaner trick). I am getting bad gas milage too.I am now a little stumped and wondering if the EGR valve is bad or maybe the O2 sensor...What do you think!!!

 

Disable the egr valve by plugging the vacuum hose that goes to the valve, drive it a couple of days to see if it acts up.