Follow-Up:
I too have a 1997 Rialta and the check engine light kept coming on. It would eventually go out without having to be re-set. Seems the problem was a bad "canister purge valve" located on top right of engine. This little monster pulls gas fumes from fuel tank to prevent you from smelling gas. It got so bad it caused the engine to go into idle mode, even while going down interstate. Would do fine going up hill, but when you let off gas and then tried to accelerate it would suck all the gas out of the intake. VW dealer wanted well over 300.00 for a new valve. Anyway, I just disconnected the vacuum line from it and plugged the line with a golf tee. This stopped the engine light from coming on and increased gas mileage 2 MPG. BTW I never smell fumes with it disconnected.