idle speed increase

I have a '97 Cavalier with a 2.2 and an automatic trans. While sitting at stoplights, the idle will suddenly jump from 600 to 900 RPM for no apparent reason. At times, it has been necessary to apply more brake pressure to prevent the car from lurching forward. After a minute or so, the idle returns to normal. The car has never done this before, even when the engine fan and air conditioning are on.
It has started stalling on rare occasions when decelerating to take a corner or approaching a stoplight. It restarts immediately when the key is turned.
The car has also begun lurching during very slow, light throttle use - like when going through a parking lot.

I would like to believe that all of these conditions are somehow connected, but cannot find a common link. I've checked many times for a vaccum leak, even pinching off the hose to the brake booster, but cannot find one.

Any ideas would be greatly appreciated.
Thanks