A B737-300 FLT; WHILE ACCELERATING TO 300 KTS; UNEXPECTEDLY CLIMBS 200 FT ABOVE IT'S LEVEL CRUISE ALT AFTER SELECTING A LOVER ALTITUDE ON THE ALT SELECTOR ON FREQ WITH ZAU; IL.

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FO ABSENT FROM FLT DECK USING FACILITIES. OBTAINED ATC CLRNC FROM FL280 TO FL200 AT CLAUD INTXN AT PLT DISCRETION AND INCREASE SPEED FROM 280K TO 320K WHEN ABLE. WITH AUTOPLT ON; ALTITUDE WAS SET TO 20000 AND SPEED INCREASED TO 300K IN LEVEL FLT. AIRPLANE ENTERED AN UNCOMMANDED CLIMB WITH THE AUTOPLT STILL ENGAGED. IMMEDIATELY UPON NOTICE OF CLIMB; AIRPLANE PLACED INTO IMMEDIATE DESCENT; AND THROTTLES RETARDED TO IDLE MANUALLY. DESCENT CONTINUED TO FL200 AT CLAUD AND SPEED THEN INCREASED TO 320K AS ABLE FO RETURNED TO FLT DECK JUST AFTER DESCENT INITIATED; AND ATC INFORMED OF LEAVING FL280 FOR FL200 WITH FOR PRESENT. SUPPLEMENTAL INFO FROM ACN 524022: APPROX 48 PLUS HRS AFTER OUR FLT TOGETHER; I RECEIVED A PHONE CALL FROM THIS PARTICULAR CAPT. TOLD ME THAT ON OUR FLT TOGETHER; HE MAY HAVE DEVIATED FROM FL280 BY 200 FT HIGH JUST PRIOR TO OUR DESCENT WHILE I WAS USING THE BATHROOM AND ABSENT FROM THE COCKPIT. HE ALSO SAID THAT AT THE TIME OF THE OCCURRENCE; HE FELT IT WAS A NON EVENT AND THEREFORE; NEVER BOTHERED TO BRIEF ME UPON MY RETURN TO THE COCKPIT. LATER HOWEVER; HE CHANGED HIS MIND AND FELT THAT WE SHOULD BOTH SUBMIT A RPT ON THE EVENT THAT HAPPENED.