THE PLT PLANNED A NONSTOP FLT FM WA TO CA, & HE RECD PREFLT WEATHER BRIEFINGS SHOWING GENERALLY VFR CONDITIONS. ENROUTE, THE PLT AMENDED HIS ROUTE & LANDED AT KLAMATH FALLS, OR. DURING ROLLOUT THE ACFT NEARLY VEERED OFF THE SIDE OF THE RWY. THE PLT TOLD ATC HE NEEDED A REST STOP & HE HAD HAD A FLAP PROBLEM. WITNESSES REPORTED THE PLT LOOKED "NERVOUS, UNDECIDED & BEWILDERED." THE PLT CONTINUED HIS S-BND CIRCLING ROUTE OF FLT WHICH PLACED TEH ACFT OVER THE WESTERN SLOPES OF THE SIERRA NEVADA MTNS AT ELE OVER 7000 FT MSL. 40 MI WEST LAY THE CENTRAL VALLEY OF CA, ELE 300 FT MSL. WHILE CRUISING AT 13,000 FT THE PLT REVERSED COURSE & DISAPPEARED FM RADAR. THE ACFT CRASHED INVERTED INTO 4900 FT MSL TERRAIN, ABT 1.5 FM ITS LAST RADAR POSITION. WITNESSES REPORTED IT WAS RAINING HEAVILY & LIGHTNING WAS VISIBLE. THE PLT HAD NOT LOGGED ANY INST FLT TIME IN 19 YRS. HIS TOTAL TIME WAS 88 HRS. HIS TOTAL PIC TIME IN THE ACFT (WHICH HE HAD PURCHASED 3 WKS BEFORE THE ACCIDENT) WAS 0.5 HRS.