Check the head bolts. You may have run it hot a time or two and the bolts do stretch with the heat. The tip off is brown stuff around the head or on the head. Just had a new Magum .28XL and after the first two runs had a bunch or brown castor on the head. Wiped it off, tightened the head bolts (alot) and problem fixed.

Also, your description sounds suspiciously like a too lean setting that causes the engine to heat up and lose power as the flight goes on. Maybe that's why you need to let it cool down. What you can try is pulling the throttle back to 1/2 to 3/4 which causes (usually) a richer setting. When I'm losing power, its the first thing I do and the engine usually comes back in a half a minute or so.

Good luck!