There is some dark brownish stuff that the factory put around the throttle body bore right where the butterfly plate contacts when it is fully closed to help create a better seal. That stuff is called DAG. It's easily mistaken for dirt/oil. I don't think you can get it anymore and to my knowledge, no one has found a suitable substitute yet.
Since you used TB cleaner on it, you probably cleaned off the DAG and now the butterfly plates don't have that seal anymore.
Here is a link to a page on Marc Haibeck's site where he talks about the idle issues the LT5's TB can develop:
http://www.zr1specialist.com/HAT%20W...tle%20body.htm