I have had similar problems with my '99 SVT. I have cleaned the IAC, and replaced the IAC and the problem does seem to reoccur after a time. Resetting the PCM seems to help for a while. I have also observed that the problem seem to crop up more in hot weather, especially after using the AC on a longer highway drive. And, no, the TH fix doesn't make any difference on this. I have tried it with the plug both in and out.
I also believe that this is a firmware issue in the PCM. My only solution so far has been to reset the PCM.
// Chris Root
// '99 SVT
