horrible startup & idle

Here are the problems I'm seeing:
start the car...rpms jump around a bit and either drop until the car turns off or I can press the gas and they'll hang dramatically for a while and then do the same...a few presses of the gas for about 20secs generally cures the low rpms....but even a 10-15 minute drive doesn't cure the hanging rpms. Every time I start my car cold I get the same issue...wouldn't bother me much but my exhaust isn't pleasing the neighbors at 6:00 am when I'm revving it 5-6 times to get the engine running smoothly

Latest things I've done:
just finished reinstalling the s/c...meaning i had taken off the TB and plenum to swap back to the bigger versions I used to have...car never ran this badly before. I've checked the tps a week ago and it was bad (problems were different then)...long story short, it has been replaced...the mongoose tune didn't take the first two times I loaded it (not sure why)...I drove and even though I set the rear gear ratio, the speedometer was way off...the third time I programmed the car, it appeared to accept the 4.10 gear ratio for speedometer adjustment

what do I need to look at?
do I need to recalibrate my new TPS??? Is there a possibility that my X-cal 2 isn't loading the mongoose tune correctly??? Does this seem like an IAC valve problem or something more serious???

any help would be greatly appreciated

thanks for the replies...as far as the 'hanging rpms'...this is worse than I've personally ever seen it...say I rev the engine upto 3000...it'll sit there for about 3-4 secs before it starts to come down....now when I hear you guys talking about 'hanging rpms', I generally would think that just means a slow descent...not an actual hangup and then slow descent...any opinions?

and what should I look at as far as the IAC is concerned...possibly just take it off and look for something out of place, or pay the $$$ for a new one???