Sputtering engine

I have an 89 corolla, and the engine continues running and sputtering occasionally for a few seconds when I turn off the engine (vibrating at low rpm as it does when you stall the engine). I have been told that it may be carbon deposits on the piston heads acting like little glow plugs, or the ignition timing. I have ran about 4 bottles of carbon removing additive through the engine and it doesn't really seem to have made a significant difference. I haven't checked the ignition timing, but I checked the distributor and there were no signs of it having rotated (it didn't do this sputtering until I put an additional 6000mi on the engine). Is there anything else that could be causing the problem? The light for the throttle position sensor has been on, and i've noticed that the engine revs higher than 750rpm or so at idle--could this be the problem?
Any help would be appreciated,
Thanks,
NICK
p.S. Do you recommend any additives--perhaps the ones I used aren't that good. I have run into some that are gasoline additives, and some that you poor directly into the carburetor intake while gunning the engine--which works best?
Thanks!