TWO PLTS & 2 PASSENGERS DEPARTED SAN ANGELO, TX, ON A X-COUNTRY FLT, ONE OF THE PLTS, WHO HAD 95 HRS OF FLYING TIME, WAS THE PILOT-IN-COMMAND (PIC) FOR THE 1ST LEG OF THE X-COUNTRY TO EL PASO, TX. ON THE NEXT LEG, THE OTHER PLT ASSUMED DUTIES AS PIC & SIGNED THE FLT PLAN. DURING AN INTERSECTION TAKEOFF WITH 5600 FT REMAINING, THE ACFT LIFTED OFF AFTER ROLLING AN ESTMATED 1500 FT. IT THEN CLIMBED TO AN ESTIMATED MAX ALT OF ABOUT 300 FT. THE PLT ACCEPTED A TURN TO 120 DEG TO MAKE WAY FOR A DEPARTING JET. AFTER STARTING THE TURN, THE ACFT SUDDENLY DROPPED, LOSING ABOUT 1/3 OF ITS ALT. A WITNESS REPORTED THAT IT CLIMBED & DROPPED 2 MORE TIMES. ON THE LOST TIME, IT CRASHED IN THE DESERT IN A STEEP DESCENT. NO PREIMPACT/MECHANICAL PROBLEM WAS FOUND. THE PROPS HAD EVIDENCE OF HI ROTATIONAL DAMAGE. THE DENSITY ALT WAS ESTIMATED TO BE 6424 FT. TOWER PERSONNEL DESCRIBED THE ACFT AS BEING NEAR ITS STALL SPEED FROM LIFT-OFF UNTIL IT CRASHED. THE ACFT WAS ESTIMATED TO BE 38 LBS OVER ITS MAX GROSS WT LIMIT.