We have 3 older STIHL trimmers that are used every day. In the middle of the day they all start acting up. They keep dying over and over. We just had the carburetor's rebuilt, new spark plugs, and new filters installed. It's like they get too hot and just die. Gets really annoying when you're trying to work. Any idea what the problem could be? Weird because they work fine for a few hours, then they start acting up.