If it makes you feel better, a buddy of mine broke the crank on his motor after suffering similar conditions for several months. Made for some cool pix, though.
Of course, he was burning oil like crazy, and I suspect he was having a lot of preignition from the heavy carbon deposits. He could hear the detonation, but didn't listen about taking it easy on it...
I suspect a fuel delivery issue. Fuel pump may be reducing fuel delivery once it gets heated up/has a decent load on it. Do you run it low on fuel a lot?