If the rad cap is bad, it might not pick up coolant from the overflow tank as the engine cools down.
This means that each time the engine heats and cools there is a little less coolant in the radiator. Eventually you have a low coolant situation, and an engine that overheats.
I was about to suggest this since that was the propblem in my SRT4 and it took close to 30000 miles before it showed up. Check your radiator (not overflow) level. If it is low, think about refilling it and getting a new cap.
Tell Lycoming, Contential and Honda that flat opposed engines do not last well and are considered not dependable. Throw Volkswagen and Porsche in there too.
YMMV