THE PLT WAS ALERTED AT 0100 FOR 0200 FLT FM KENOSHA TO MILWAUKEE, WI TO PICK UP CARGO FOR A FLT TO ATLANTA, GA. AT 0310,THE ACFT DEPTD MILWAUKEE IN VFR. EN ROUTE, THE PLT OBTAINED WX & AN IFR CLNC. SOME OF HIS RDO XMSNS WERE NOT STANDARD. AT 0433, HE COMPLIED WITH ATC REQUEST FOR FREQ CHG, BUT HIS RDO XMSNS WERE NOT CLEAR. APRX 5 MIN LATER, ATC ASKED THE PLT TO CHG FREQ AGAIN, BUT 5 CALLS WERE MADE BFR HE REPLIED. HIS LAST XMSN WAS AT 0445:48; HE DIDN'T RESPOND TO FURTHER ATC CALLS. THE ACFT CONTD CRUISING AT 11,000' UNTIL 0621, THEN IT DSCNDD W/O CLNC. RADAR CONTACT WAS LOST AT 0627 CST (0727 EST). SHORTLY THEREAFTER, IT IMPACTED TREES ON A MTN, THEN CRASHED TO THE GND & BURNED. A WITNESS BELIEVED THE ENGS WERE THROTTLED BACK BFR IMPACT. ALSO, THE WITNESS RPRTD LOW CLDS & FOG, BUT SAID THE MTN WAS VISIBLE & WAS NOT OBSCURED. THE PLT HAD A 0.158% BAC, NO SLEEP FOR 21.5 HRS, HISTORY OF ALCOHOL ABUSE, 7 PRIOR DWI CONVICTIONS (& NDR REC). ALSO, HE FALSIFIED FAA MEDICAL APPLICATIONS. FAA WAS ADVISED IN 1984, BUT TOOK NO ACTION.