As the pilot reduced power on reaching cruise altitude, he noticed a vibration through the aircraft. The vibration increased in intensity and the aircraft instruments became difficult to read. The pilot was unable to determine the cause of the vibration and decided to carry out a landing in a paddock. The downwind approach was misjudged and the chosen landing area overshot. The pilot then became concerned about the proximity of the trees at the end of the paddock and attempted to force the aircraft onto the ground. The aircraft struck a tree, then the ground, nosed over and came to rest inverted. During the approach the pilot had not used flap as he had been concerned that the vibration may have been airframe induced. However, inspection of the aircraft did not reveal any contributory airframe fault. Evidence was found that the four exhaust valves had been sticking and given the high ambient temperature, high oil temperature and low oil pressure indications subsequently reported by the pilot, it is probable that the vibration was caused by stuck exhaust valves.