Well I have a 91 GMC with a 350. When I bought the truck it had a rough idle in cold weather. ( I live in MT). Well I had a brand new 350 installed by a mechanic in my truck and I still have the same problem and also now it rough idles when the motor is hot. I had the idle control replaced and nothing. Like I said the motor itself is new. And the bad idle is intermittent.

 

Look for this...
at your thermostat housing area are 2 grounds.....
one ground goes to the housing stud..THE OTHER TO AN INTAKE STUD...the 2 CAN NOT share the same stud....after many repaitrs,techs put em both on one stud,and she'll do stupid stuff

 

There is only one on the housing. Any other ideas?