While in cruise flight at 6,500 ft, at a power setting of 2500 RPM with 21 inches MP, the pilot stated he was unable to synchronize the left prop. He stated the prop then decreased to 1500 RPM, surged to 3000, then dropped back to 1500. He attempted to feather the prop but was unable to do so. Unable to maintain altitude with power from the right engine, the pilot made a forced landing on an interstate. During the landing roll, the left wing contacted an embankment and the aircraft veered off the side of the interstate. A fire started in the right engine and the right wing was destroyed. Post accident inspection of the left propeller failed to reveal any reason why the fluctuation in RPM had occurred. Nor did it reveal why the prop would not feather. Water was found throughout the left engine fuel system. The props had been overhauled prior to this flight. The left prop governor was bench tested and although capable of operation was found to be operating slightly out its specified operating range.