1996 Lumina 3.1 idling surges

My 1996 Lumina 3.1 (182K) seems to have a mind of its own. On warm days travelling at 80 kph, the car gets locked at 80 kph (2000 rpm) and I have to struggle with the brakes to slow it down, and in the process, sometimes the engine shuts off. It however restarts readily. No problem on highways. No codes have appeared. The guys at the garage first disconnected the cruise control, but problem is unchanged. Then they replaced the IAC motor...still no improvement. I have now learnt to control the car better by shifting to neutral when the condition occurs, letting the rpm shoot up to 3500 for about 20 seconds before it settles back to 900. This is sometimes a tricky manuovre, and my wife dreads to drive the car.I am looking for a sure fix - need help badly. Any suggestions?
ennay

Interesting, my '97 did the exact same thing a few months back, except that the motor did not die, but it was fighting me to a stop, and trying to run along at about 40 mph with my foot off the gas.

I got on the highway and gave it the spurs all the way home, and it was fine after that....hasn't done it since.