On August 19, 2000, at about 1058 eastern daylight time, a Ryan ST3KR, N54403, registered to GVS Corporation, operating as a 14 CFR Part 91 personal flight crashed during a forced landing in the vicinity of St. Lucie, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The airplane sustained substantial damage. The airline transport rated pilot-in-command (PIC), and commercial pilot-rated passenger reported no injuries. The flight originated from Fort Pierce, Florida, about 28 minutes before the accident. The PIC stated they were in cruise flight at 1,000 feet when he experienced a partial loss in engine power (rpm decreased to 1,000 rpm). He immediately turned on the carburetor heat, the rpm increased, and he continued with the flight. About 5 minutes later, he experienced a second partial loss of engine power and elected to make a forced landing to a highway. On final approach he observed power lines and cleared them. On touchdown the right wing collided with a tree at about mid wing. The airplane rotated to the right and turned over inverted. The pilot stated in the NTSB pilot/Operator aircraft Report that after lining up with the road he observed numerous automobiles facing him. He applied right rudder to avoid traffic when the right wing collided with a pine tree and a sable palm tree. The airplane yawed to the right followed by the left wing impacting another tree. The fuselage rolled over up against a tree inverted. Examination of the engine assembly could not determine the source of the reported loss of engine rpm. Fuel was present in the fuel tank, and the fuel tank vent was clear and the fuel cap was properly vented. No fuel was present in the gascolator, and the drain valve was found in the off position. The fuel selector valve was found in the reserve position and functioned normally when activated. The fuel strainer contained about 6 ounces of fuel and did not appear to be water contaminated. The fuel inlet screen on the carburetor was clean and the carburetor float bowl did not contain fuel. Compression and valve train continuity of the engine was established on all cylinders, and the impulse coupling equipped magneto supplied spark to all cylinders at the forward spark plugs. The aft row of spark plugs could not be checked due to the limited rotational speed provided by hand cranking. (For additional information see FAA Aviation Safety Inspector statement, an attachment to this report.)