DC9 CREW HAD UNCOMMANDED FLAP EXTENSION; DURING CRUISE.

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IN CRUISE AT FL330 AFTER ABOUT 30-45 MINS; I NOTICED THAT IT WAS TAKING SIGNIFICANTLY MORE PWR (+.10 EPR) TO REMAIN AT CRUISE MACH (MACH .75) THAN SCHEDULED. I NOTED THAT THE FLAP INDICATOR SHOWED 5 DEGS FLAP EXTENSION; HANDLE UP; SLATS RETRACTED. I SENT THE FO BACK TO VISUALLY CHK FLAP POS AND HE RPTED THAT THE FLAPS APPEARED TO BE EXTENDED 5 DEGS. I THEN WENT BACK TO SEE; AND ALSO FELT THEY WERE EXTENDED TO AT OR NEAR 5 DEGS. I REQUESTED A LOWER ALT FROM ATC AND WAS CLRED INITIALLY TO FL250. I KEPT THE AIRSPD AS LOW AS SAFELY POSSIBLE DURING DSCNT UNTIL WE REACHED AN ALT WHERE THE FLAP/MACH AIRSPD LIMITS WERE NO LONGER EXCEEDED. WE INITIATED A DIVERT INTO DTW. AT 12000 FT; I NOTED THAT THE FLAPS APPEARED TO HAVE RETRACTED ON THEIR OWN. AT NO TIME DID WE MOVE THE HANDLE OUT OF THE 'UP' DETENT DURING THIS EVENT. ON APCH; LNDG AND TAXI IN; THE FLAPS EXTENDED AND RETRACTED NORMALLY. THE DC9 HAS OLD; ELECTRO-MECHANICAL TYPE FLAP INDICATORS AND IT IS COMMON FOR THESE TO NOT READ 'EXACTLY' ON THE SELECTED FLAP POS. MOST DC9 PLTS LEARN TO IGNORE INDICATIONS UP TO SEVERAL DEGS OFF OF WHAT IS SELECTED. IF EXTRA PWR HAD NOT BEEN REQUIRED (AND MY SUSPICIONS RAISED) I MIGHT NOT HAVE NOTED THE 5 DEG INDICATION AT ALL AS BEING ABNORMAL.