I have an 89 w/ a 3.1 swapped in and a T5. when I first started it up after sitting in my shop for a week or so it wouldn't idle at all as in I kept my foot on the gas to keep it running. after taking it around the block and getting it up to temp it idled smoothly, but kinda high, around 1200 RPM. I started it up yesterday and it wouldn't idle the first minute or so, but then started idling to 600, then kicking up to 1500 like it couldn't decide where to idle. off idle it runs just like it should. I'm 99% sure I don't have a vacuum leak. to me it sounds like a sensor, maybe IAC valve messed up somehow?
usually it idles around 2000 until it gets warmer, then it idles down. I'm pretty sure thats how its supposed to work. the idle speeds were found in neutral. (by the way, does anyone else love their heater as much as I love mine?)

 

I thought I should mention during my drive the check engine light didn't come on.

 

mmmm the idle value is stuck u check the timing does the car feel slow or it bogs at speeds above 20mph

the oem idle is 650rpm i think or v6 3.1

 

the timing shouldn't be out of whack, it doesn't bog. unless your trying to go like 30 in 5th gear. but it still goes fine, just slow.
another possible problem could be the fuel pressure regulator. thats no big deal, I was going to install a rrfpr in the near future.
the lowest my car has ever idled is around 700, so its not far off. I'm not going to tear into anything right now because there is a big snowstorm going through and its waaaaay too cold to be outside for long. besides, Its not on the road until april. this seems to be cold related. its never had this kind of problem before so temperature is my prime suspect.