2000 Cavalier Idle Surging High and Low

PLEASE HELP!! I can't afford to take this car to the stealership and am honestly hoping that someone with engine knowledge will be willing to help me out.

Symptoms....car was driving down the highway just fine, exited the ramp, rolled up to the stop light and the thing just started idiling high, around 3000 rpm. I was pushing the brake as hard as I could to get the car to stay in place or else it would take off like a shot. The idle was surging from low to high and back down over and over again so I pulled into a parking lot and shut it off because I didn't want it to hurt the motor. I let it sit for about 2 days and just tried it again, this time it's idiling high, somewhere between 1500 - 3000rpm. What could be the cause of this? Thank you very much in advance........oh, it's a 2200 SFI 4 cyl. engine with approx. 144,000 miles and automatic tranny.

it's most likely the IAC valve, but could also be a large vacuum leak

Thank you so very much! I have been doing some investigating and those seem to be the most common "fixes", so will start with those.

it could also be and intake gasket leak if the gasket shifted and created a leak the idle would bounce around but would idle really high. I have had it happen to me once it took me 3 days to figure out it was the damn gasket i jusy replaced but had shifted. i would start small with the vacuum leaks if the IAC valve went bad it would more than likely barely run