Pretty strange for two cars to have the same issue at the same time. I have to wonder if the problem isn't gas related and the ballast was just a fluke that gave the gas issue enough time to run a bit more. Have you bought gas for both at the same time recently? maybe you got some bad gas the last time. The next thought is about the opposite, the current swill they sell as gas doesn't have a very long shelf life anymore, is the gas in both cars more then 6 months old?
It was strange that both cars did that. I've been using race gas in all the cars for a few years now, since they don't get driven too often.
