my mower does the same thing, but it doesn't bother me, and i read it was most likely a bad coil, or the valves were not seating correctly or something.
Sometimes a coil that's on it's way out will quit working when it's hot, and then resume functioning when it cools off. I've seen it happen a couple times, I just don't understand why. Perhaps temperature can affect resistance values. Take a cold reading across the coil, then run it 'till it quits and re-read immediately...