The pilot took off with full fuel tanks and flew for about 2 hours 15 minutes before the accident occurred. The airplane was observed near the accident site, flying erratically beneath a 300 to 400 foot ceiling in strong, gusty wind conditions. Witnesses reported the engine seemed to be cutting in and out. The airplane was northbound with a 25 to 40 knot tailwind when it was observed to pitch up abruptly, rotate left, and nose down into the ground. Post-accident examination of airframe, powerplant and systems revealed no evidence of pre-impact failure/difficulty. The left fuel tank was nearly full of fuel. The right fuel tank was compromised at impact, however there was no evidence of fuel spillage at the accident site. The landing gear was extended, fuel boost pump was on, and the fuel selector was jammed in an intermediate position. Fuel consumption calculations (using 75%) indicated as much as half the available fuel may have been burned.