The aircraft was cruising at 6500 feet; the mixture had been leaned and the engine was operating normally. The pilot then reported that the engine note changed although all engine indications were normal. Approximately five minutes later the engine coughed and spluttered and started to lose power. The loss of power was reported to be rhythmic in nature, a few seconds of power followed by a like period of loss of power. The pilot turned the aircraft towards the DEPARTURE point and completed trouble checks, but these checks did not rectify the malfunction. It became apparent that the DEPARTURE strip could not be reached so the pilot turned south to intercept a dirt road he knew was in the area. The dirt road was located and with the stall warning sounding he selected full flap and turned to line up with the road. During the turn the aircraft grazed the top of a seven metre high tree. The aircraft then stalled onto the road from a low height. The nosewheel collapsed and the aircraft nosed over onto its back. Since there was no suitable terrain for a forced landing within gliding distance an accident was inevitable. Despite extensive examination of the engine and its components, no defect that would have caused a partial or total engine failure was found.