Just remembered that I had a problem with my old 95 TSi and a stuck high idle. Mitsu never figured it out and a new ISC didn't fix it. Eventually I found that the throttle cable had a very small burr in it that didn't allow the throttle plate to close all the way. This only happened on hot/humid days in Iowa. I only figured it out because it eventually one day decided to stick permanantely while I was driving about 70mph on the highway and would not slow down, threw it in neutral and it redlined! New throttle cable = problem solved.
I don't think that is your problem cause I can't see an electrical current screwing with that cable, but it may be worth checking into.