My pickup has a new Holley TBI unit, and has had prior idling issues, which turned out to be the TPS. After replacement, the truck ran like a top until cold weather. Now it tends to idle either too high(900rpm) or too low(500rpm), depending on what mood she's in. It drives me nuts, because she's also a stick, and that can make take off tricky at times. This problem is also amplified when the defrost is on. Once the A/C compressor kicks in, the idle increases then levels off, as it should, but after the first cycle it just bogs the engine down to 500 rpm or sometimes kills it. I think it might be the IAC, but I was hoping for advice before I started just replacing stuff.

Thanks.