Jlip,
I had a very similar thing happen to one of my cars. After it got hot it was hard to start and just ran really bad. I disconnected the MSD unit and I hadn't gone 50 feet before I cold tell the difference. I think the heat caused a failure in mine as I never bothered to contact them to see if they would replace it. I went to another brand and have been much happier with it. I think MSD quality has gone way down in the past few years.
Ron
