WHILE EN ROUTE ON A NIGHT IFR CARGO FLT, THE PLT REQD & WAS CLRD FOR A DSCNT FROM AN IFR CRUISE ALT OF 16,000 FT TO PROCEED VFR-ON-TOP. THE LAST TRANSMISSION FROM THE ACFT WAS AT 2106 WHEN THE PLT ACKNOWLEDGED THAT HE WAS FAMILIAR WITH THE TERRAIN ALONG THE ROUTE. AT 2121:51 MDT, RADAR CONTACT WAS LOST AFTER THE ACFT MADE AN APRX 360 DEGREE LEFT TURN & DESCENDED THRU 13,800'. SHORTLY AFTER THAT, THE CONTROLLER TRIED TO CALL THE PLT TO INFORM HIM THAT RADAR CONTACT WAS LOST, BUT WAS UNABLE TO MAKE RADIO CONTACT. LATER, WRECKAGE OF THE ACFT WAS FOUND WHERE IT HAD CRASHED ABOUT 2 MI SW OF WERE RADAR CONTACT WAS LOST. THE WRECKAGE WAS SCATTERED OVER AN AREA OF MORE THAN ONE MILE. AN INVESTIGATION REVEALED THE LEFT WING & RGT HORIZONTAL STABILIZER HAD SEPD UPWARD FROM OVERLOAD. NO PRE-ACDNT FATIGUE OR PART FAILURE/MALFUNCTION WAS FND. WX RPRTS & PIREPS INDCD MODERATE TURBC & TOWERING CUMULUS CLOUDS IN THE AREA. APRX 60 MI WEST AT POCATELLO, ID, SNOW SHOWERS WERE RPRTD; 2 TO 3 INCHES OF SNOW FELL ON THE WRECKAGE OVERNIGHT.