It sounds like the compression seal of your engine is starting to go. While the engine may feel like it has "good" compression when it's propped on the ground, in the air the engine's heating up and the parts are expanding enough that the engine's losing compression. Likely replacement of the ring, and possibly the piston, too, will get back your engine's performance....unless the valve seals are also starting to go. With the amount of use you mention, it's not unlikely that the engine's just wearing out.