B737 CREW HAD AN ALTDEV.

Narrative

CAPT FLYING IN CRUISE AT FL330. 'A' AUTOPLT; LNAV AND VNAV WERE ENGAGED. FL330 WAS SET IN MCP WINDOW. '250 KTS' AND '10000 FT' WERE PRESET ON DSCNT PAGE IN FMS FOR HOAGI INTXN. WE DID NOT HAVE DSCNT CLRNC YET. ACFT BEGAN AN UNCOMMANDED DSCNT WITH FL330 STILL SET IN MCP WINDOW. CAPT WAS BRIEFING APCH AT THE TIME AND I WAS LOOKING AT APCH PLATE. THE DSCNT COULD NOT BE FELT; BUT I NOTICED THE THROTTLES RETARDING AND REALIZED WE WERE DSNDING JUST AS THE ALT ALERTER WENT OFF AT FL327. BOTH OF US REACHED FOR THE CTL WHEEL AND THE DSCNT WAS STOPPED BY FL326. WE IMMEDIATELY RETURNED TO FL330. PRIOR TO THIS FLT; 'A' AUTOPLT HAD BEEN WRITTEN UP AS 'NOT HOLDING ALT.' IT WAS; HOWEVER; SIGNED OFF AS FIXED BY OUR MAINT DEPT. THE CTLR NEVER COMMENTED ON THE ALT LOSS AND THERE WAS NO TFC CONFLICT. SUPPLEMENTAL INFO FROM ACN 424254: PRIOR TO THIS FLT; 'A' AUTOPLT WAS WRITTEN UP AS 'NOT HOLDING ALT.' BOTH MAINT WRITE-UPS HAD BEEN WRITTEN OFF BY OUR MAINT DEPT. 'A' AUTOPLT HAD BEEN FUNCTIONING PROPERLY FOR APPROX 2 HRS. ZHU DID NOT COMMENT ON THIS 400 FT ALT LOSS.