THE PLT WAS FLYING INBOUND FOR THE ILS RWY 02 APCH VIA THE GARDNER VOR TRANSITION. WHILE ON THE TRANSITION, THE ACFT DESCENDED BELOW THE 4000 FT MSL MIN ALT TO 2600 FT MSL. THE PLT WAS ALERTED BY ATC. HE RESPONDED THAT HIS CHART READ 3000 FT. THE ACFT THEN CLIMBED ABOVE 2700 FT, BUT DESCENDED AGAIN (TO 1500 FT MSL) UNTIL ABOUT 2 MI OUTSIDE THE OUTER MARKER. THE MIN PUBLISHED ALT TO INTERCEPT THE GLIDE SLOPE WAS 2600 FT & AND THE RECOMMENDED (GLIDE SLOPE) ALT FOR CROSSING THE OUTER MARKER WAS 2548 FT. BEFORE REACHING THE OUTER MARKER, THE ACFT MADE AN ABRUPT TURN TO THE SOUTHWEST, ENTERED A STEEP DESCENT & CRASHED IN A WOODED AREA. AN EXAM OF THE ACFT DID NOT DISCLOSE ANY EVIDENCE OF A MALFUNCTION. A FLT INSPECTION OF THE NAV AIDS DISCLOSED SATISFACTORY OPERATION. A POST MORTEM EXAM WAS INCONCLUSIVE TO POSSIBLE PHYSICAL IMPAIRMENT. THE PROCEDURE TURN ALT FOR THE APCH WAS 3000 FT, BUT THE GARDNER TRANSITION DID NOT REQUIRE A PROCEDURE TURN.