The pilot reported that, shortly after transition to cruise flight, he noticed that the engine was "running rough," the cylinder head temperature was almost in the red, and the engine rpm and manifold pressure were decreasing. He decided to return to the departure airport for a precautionary landing. On approach, he decreased the engine power setting and increased the mixture and propeller settings, and the airplane then crossed the runway threshold about 80 knots with full flaps. The pilot added that, during the landing flare, the airplane "dropped" and then bounced two to three times on the runway. He taxied the airplane off the runway, and it was then was towed back to the ramp. The airplane sustained substantial damage to the fuselage. The Federal Aviation Administration inspector reported that there no mechanical failures or malfunctions with the airplane that would have precluded normal operation. The airport's automated weather observation station reported that, about the time of the accident, the wind was variable at 4 knots. The pilot landed the airplane on runway 33.