Intermittant Rough/Serging Idle

[nitrosport_5]

been trying to fix this problem for quite a while, still have not come up with a solution. I thought that it only did it when it was hot outside, but it was doing it to me this morning after i got to class.

Its a 92 k1500, 210,000 miles. has a fairly recent gm crate 350 (about 30-40k). I beleive the throttle body was replaced when the engine was replaced. Working on it in the passed, i thought it was fuel pressure because if you pinched the return line, it would smooth out. Replaced the fuel pump and filter with same results, tested the fuel pressure and it came out to be in spec. Replaced the IAC valve because i had heard that could cause the surging. didnt fix it. Checked the readings of the CTS and they seem to be within spec. Had it hooked up to a snapon computer of a friends and it couldnt see anything wrong. I never have a check engine light or anything.

This morning i went out and started the truck to warm it up, was about 32* outside. Does its normal high-er idle until it builds a little heat and then idles itself down. assuming this is the choke more or less. I drive about 45 minutes to get to class, pull in the parking lot and it is idling about like its got a cam of a 1000hp big block. i can rev the engine to a normal 2k rpm and hold it steady and it runs great, let it idle back down and it will keep the rough idle. If i wing the throttle to about 4k rpm or more, it will idle normally again. after class i get back in it and it is running just fine, drive a few blocks to my apartment and shut it off.

does anybody have any more ideas? Im out of things to try on this and it seemt o be somthing that has everybody stumped. My dad was a mechanic that put the engine and trans in there and he was never able to figure it out. His old boss that is an old school mechanic was not able to figure it out. Hopefully someone else has run into this issue before and is able to help me out.