PRIOR TO THE FLT, THE PLT REPORTED THAT HE WAS 'GOING TO DO SOME AIRWORK.' ABOUT 1.2 HRS AFTER TAKEOFF, WITNESSES SAW THE ACFT FLYING EASTBOUND AT AN ESTIMATED ALTITUDE OF BETWEEN 2000 & 3500 FT AGL. A WITNESS WITH ABOUT 40 HRS OF AERONAUTICAL EXPERIENCE OBSERVED THE ACFT NOSE UP LIKE THE PLT WAS 'PRACTICING TAKEOFF TYPE STALLS.' ALL 4 WITNESSES NOTED THAT THE ACFT WENT INTO AN EXTREME NOSE UP ATTITUDE, THEN PITCHED OVER INTO A STEEP DESCENT. AS THE ACFT WAS DESCENDING, IT REPORTEDLY WAS PITCHING UP & DOWN & THE WINGS WERE ROCKING, THEN THE LEFT WING DROPPED & THE PLANE DISAPPEARED FROM THE WITNESSES VIEW. SHORTLY AFTER THAT, THE ACFT IMPACTED ON AN ELEVATED MESA AREA. THE WRECKAGE WAS SCATTERED ABOUT 438 FT BEYOND THE INITIAL IMPACT POINT. AN EXAM OF THE WRECKAGE WAS MADE, BUT NO PREIMPACT, PART FAILURE OR MALFUNCTION WAS FOUND.