1996 Monte Carlo throttle problems

I drive a 1996 monte Carlo Z34 with 263,000 kilometres on it. I've owned it for three years now, and I've only needed to replace an alternator, belt tensioner, and alternator wiring. Occasionally, my car will just randomly rev much higher than it's supposed to. Sometimes while I drive, it feels like I have cruise control on, but I don't. And when I stop and put the car in neutral or park, the RPMs shoot up to about 3000 or more. Usually the problem goes away if I let the car sit for a few hours, but it's winter now, and it happens very often. So while it was running, I pulled on the cruise control cable and the rpms went down to normal. I thought that was over, but the next day, it wouldn't start. I got the mass air flow sensor cleaned, and the car ran fairly good. But on my way home this evening, the rpms stuck at about 2000. When I stopped the car and put it in park, it shot up to 3000RPM. I slightly touched the gas, and it then went up to the rev limiter, at 4000RPM. I turned it off, but the problem keeps occurring every time I start the car. I'm thinking it could be the O2 sensor, or maybe my cruise control is sticking, but I could use some expert advice here. Thanks.