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
 
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Hanging idle is a common thing with all Mustangs. Its not really a problem, its just how the IAC is set up.

As far as RPM bouncing that can be caused by a bad IAC, but that is not the only possibility.
 
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???