2006 Supra Idle too high

had an issue this weekend when I would put my 24v in neutral from time to time the engine would idle at 1280, sometimes it would slowly come back down to 890 but a few times I had to turn off the engine and restart to get the idle to reset. I was able to get the idle to come down by engaging it into gear but dont like to do that to the tranny. Anyone experienced this? I recently installed Stargazer, but perfect pass says its not related to their system. I also put on a small prop as an experiment 13.17 x 17 could that have anything to do with it?