AFTER TKOF, THE PLT RCVD PROGRESSIVE ALT CLNCS TO FL200. WHILE CRUISING AT FL200, HE RPRTD 'MODERATE CHOP.' AT ABT 1125 CDT, HE REQD & RCVD CLNC TO FL220. ABT 3 MIN LTR, HE INQUIRED ABT CLD TOPS & SAID HE WAS 'IN A LAYER RIGHT NOW AT ABOUT FLIGHT LEVEL 200 TO 210.' SOON THEREAFTER, THE ACFT BGN ALT DEVIATIONS & WENT ABV THE ASSIGNED ALT OF FL220 (22,000'). IT THEN DSCNDD TO ABT 20,500', WHERE IT PITCHED UP TO AN ALT OF ABT 23,000'. THE ACFT THEN WENT INTO A STEEP DSCNT. SUBSEQUENTLY, AN IN-FLT BREAKUP OF THE ACFT OCCURRED AT ABT THE TIME IT EMERGED FM THE CLDS NR THE 2000 FT LVL. PIECES OF WRECKAGE WERE FND OVR A WIDE AREA. THE LEFT WING WAS FND ABT .2 MI FM THE FUSELAGE; PIECES OF THE RUDDER & STABILIZERS WERE FND ABT 100 YDS FM THE FUSELAGE. AN EXAM OF THE FRACTURES ON MAJOR STRUCTURAL COMPONENTS REVEALED FEATURES TYPICAL OF OVERSTRESS SEPN. NO PREEXISTING CRACKS WERE FND. AN AREA FORECAST HAD FLT PRCTNS FOR IFR, TSTMS, ICING IN THE VCNTY OF CONVECTIVE ACTIVITY. THE PITOT HEAT SW WAS FND IN THE 'OFF' PSN. (SEE: NTSB/SIR-92/03)