Im having similar issues with my 01 GTP. Just going down the road with my foot off the gas the throttle has a mind of its own. When i put it in park the rpms usually shoot way up... weirdest thing! Brought it into the mechanic today who cleaned out the throttle body. Now it idles where it should at about 900rpm instead of 1200rpm like it used to but that didn't fix the problem. Did you ever find a solution? My mechanic said its probably the tps or iac.