ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT IN A HOLDING PATTERN.

Narrative

WE WERE IN THE HOLDING PATTERN WAITING FOR THE ATL ARPT; WHICH WAS CLOSED DUE TO TRW TO OPEN. WE WERE GIVEN A DSCNT CLRNC FROM 22000 TO 17000'. DUE TO THE SHIFTING WX AND THE CHANGE IN ALT; WE FOUND OUR HOLDING PATTERN WAS NO LONGER HABITABLE. THERE WAS A BUILD UP AT OUR FIX AND ONE SITUATION SO AS TO CONFLICT WITH OUR LEFT TURN IN THE HOLDING PATTERN. THE CAPT CONFIRMED ALT CAPTURE ON THE FMA AT 17000'; THEN DIRECTED HIS ATTN TO THE ARR CHART. WE REACHED OUR ALT AND THE TURNING POINT FOR THE OUTBND HDG APPROX THE SAME TIME. THE F/O WHO WAS FLYING; CONFIRMED THE CAPTURE MODE AND THEN WENT OUTSIDE TO MAKE HIS TURN SO AS TO AVOID AS MUCH OF THE WX AS POSSIBLE. AT THIS POINT ATC ALERTED US WE WERE HAVING AN ALT EXCURSION. THE CAPT LOOKED UP AND SAW THE ALT AT 16500'. THE CAPT ALSO NOTED THE ALT PRESET WINDOW SHOWED 10000' INSTEAD OF 17000'. IT HAD DISPLAYED 17000' WHEN THE 18000' (1000' PRIOR TO LEVEL OFF) HAD BEEN MADE. WITHIN SECS THE F/O LOOKED UP TO SEE THE DISPLAY AT 7000'. THE ALT PRESET WAS RESET TO 17000'. THE ACFT LEVELED OFF AND A HDG OF 190 DEGS PER ATC INSTRUCTION WAS ESTABLISHED. IT WAS ASSUMED AT THAT POINT WE HAD BRUSHED THE ALT PRESET KNOB WHILE THE AUTOPLT WAS IN THE CAPTURE MODE; CAUSING THE ACFT TO FAIL TO LEVEL OFF. HOWEVER; LATER IN THE SAME FLT; WHILE CRUISING AT 12000'; THE ALT PRESET CHANGED FROM 12000 TO 11000'. NEITHER PLT WAS PHYSICALLY NEAR THE PRESET KNOB. I HAVE BEEN ON THE ACFT FOR 4 YRS AND HAVE SEEN THE AUTOPLT FAIL TO CAPTURE ALT FOR A VARIETY OF REASONS. I HAVE NEVER SEEN THE ALT PRESET CHANGE. TWO OF THE THREE UNEXPLAINED CHANGES OF THIS FLT COULD NOT BE ATTRIBUTED TO PLT INPUT. IF THIS IS A FAILURE THAT CAN OCCUR; IT IS QUITE INSIDIOUS AS ALT ALERT IS ALSO CHANGED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE ACFT WAS CHKED BY COMPANY MAINT AND THE PROB COULD NOT BE DUPLICATED. THE RPTR WAS REALLY NOT SURE WHAT HAPPENED; BUT DID ADMIT IT WAS POSSIBLE FOR THE PROB TO BE FLT CREW RELATED. RPTR SAID HE NEVER SAW THE ALT ALERT SET AT 7000' AS CLAIMED BY THE F/O. THE INSTRUMENT SELF TEST SYS WAS DISCUSSED; BUT NOT THOUGHT TO HAVE BEEN A FACTOR IN THE ALT DEVIATION. THE RPTR WAS NOT VERY CONVINCING ABOUT ALT PRESENT CHANGES OCCURRING W/O FLT CREW INPUT.