IAT would cause a bad and or a hard shift, but so can a MAF. I had both become an issue on mine. When my MAF went, the car stalled on the way to work. It would only start and run for about 5 seconds, then stall out again. I unplugged the MAF and plugged it back in after the car was running. When I shifted into drive, The tranny shifted so hard I thought somebody rear ended me (yeah I know how that sounds
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My point, it could be any one of the many sensors on the car causing an issue. I would suggest checking the tranny fluid first. It's cheap and easy. Just a real PITA to check as there's no dip stick. The car has to be on a level surface, warmed up, and removed the 11mm plug on the pass side of the tranny to see if anythig comes out. If anything comes out, it's full. If nothing comes out, fill until something does. Much easier with 2 people, even if youre the one doing all the dirty work and the other is just looking for fluid coming out.
See if you can get a scan done with a scanner that shows hidden codes. The hidden codes are codes that are going to come up, but right now, they are kinda like in a Beta mode. Sounds kinda weird, but I've heard GM vehicles do this a lot.
Where an IAT is a cheap sensor to pick up, theres no use throwing money at a problem that isn't there
