This seems to make the most sense. Once the sensor warms it goes open and the ECU doesnt' get a reading from it. Cool back down and it's fine. I've seen this happen with ignition coils on snowmobiles and stuff over the years - runs fine for 10-20 minutes then no spark. Let it cool and it's fine againI like Robby's theory, but I'll add another thing to check - spark gap. The bigger the gap, the more stress it puts on the coil as it takes a higher voltage to jump it.