Your symptoms are indicative of what we at CEG have dubbed "throttle rise." Do a search for that phrase, and if nothing else you'll find you're not alone. In my case, I experience what you describe very sporadically, but when it does happen, it's extremely frustrating. When I'm ready to bite through the steering wheel, it goes away.
People have blamed everything from burrs on the throttle cable to bad IAC's to dirty intakes and throttle bodies. Personally, I eye the IAC as the source of the problem, but it doesn't happen consistently enough for me to do a targeted diagnosis.
Hope this helps...
