OK, I have a 98 Cavalier 2.2L with the 3spd auto and 108k miles.

Lately it's been idling really high, like 900 - 1100 rpm which means that in Drive it throbs like as if you're torque braking it (obviously).

However most mornings when I put it in Drive the engine revs drop and it almost stalls. Today it actually did stall. Restarted the engine, back in drive and it was fine.

What's causing this? IAC valve clogged? Thanks in advance.

sounds like sticky IAC. try good expensive carb and throttle body cleaner. those are also known for sucking the "upper" intake gasket