AT 0515 EST, THE FLT WAS CLRD FOR TAKEOFF ON A FLT FM YPLISANTA, MI TO LOUISVILLE, KY. ABT 17 MIN LATER, THE FLT CREW BGN TO DISPLAY INDCNS OF A DETERIORATION OF THEIR ABILITY TO CTL THE ACFT. AT 1ST, THEY DEVIATED FM INSTRNS TO HOLD WEST OF THE FINDLAY VOR AT FL220. AS THE FLT CONTD & WAS CLRD TO FL270, THE CREW DISPLAYED CONFUSION ABT MAG HDGS & BASIC INSTRNS. AT 1048 EST, THE ACFT DEVIATED FM THE EN ROUTE HDG & THE WRONG HDG WAS READ BACK AFTER A HDG CORRECTION WAS GIVEN. ALSO, THE ACFT CONTD CLIMBING (TO FL291), THEN RADAR & RADIO CTC WERE LOST AT 0551 EST. THE CTLR NOTED THE PLT'S SPEECH WAS SLURRED & SOME PORTIONS OF THE CONVERSATION WERE UNINTELLIGIBLE. SUBSEQUENTLY, THE ACFT CRASHED IN A STEEP DIVE. NO PREIMPACT PART FAILURE WAS VERIFIED, THOUGH IMPACT FORCES & POST-CRASH FIRE RESULTED IN EXTSV DMG OF THE ACFT. THE ACFT WAS EQUIPPED WITH OXYGEN & PRESSURIZATION SYS. NO AUDIBLE WARNING WAS NOTED ON ATC RECORDINGS TO INDC THE CABIN ALT HAD EXCEEDED 10,000', THOUGH THE ACFT WAS EQUIPPED WITH SUCH A DEVICE.