AN ACR FLC RPTS THAT THE ALT ALERTER IN THEIR MD90 ACFT IS SUBJECT TO CHANGING THE SET ALT BY ITSELF WITH NO OTHER INPUT FROM CREW.

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AFTER DEPARTING DFW TO AUS; DURING CLBOUT THERE WAS A LINE OF DEVELOPING TSTMS S OF THE DFW AREA. WE WERE ORIGINALLY ASSIGNED FL220. IN ORDER TO AVOID THE DEVELOPING CLOUDS/TSTMS; WE REQUESTED DEV R OF OUR ASSIGNED 210 DEG HDG TO INTERCEPT THE JOE POOL DEP; WINDU TRANSITION. IN ORDER TO AVOID A LENGTHY (40-50 MI) W HDG DEV; WE REQUESTED FL250 OR FL260 TO GO OVER THE DEVELOPING LINE OF TWR CUMULUS CLOUDS. WE WERE GIVEN FL230 AND A FREQ CHANGE. WITH THE NEW CTLR WE REQUESTED FL250 OR FL260. WE WERE ASSIGNED FL240. FL240 WAS SET INTO THE ALT WINDOW AND VERIFIED BY BOTH CREW MEMBERS BOTH VERBALLY AND VISUALLY BY POINTING. I THEN WENT DOWN TO OPERATE THE WX RADAR. I SAW THE ALT ALERT LIGHT EXTINGUISHED AS WE APCHED FL240. I ALSO THOUGHT THE PF WAS CTLING THE AUTOPLT AND ASSURED THE ACFT LEVELED AT FL240. SHORTLY THEREAFTER; I HEARD THE ALT ALERT TONE. I LOOKED UP AND SAW WE WERE PASSING THROUGH FL242.5. I TOLD THE CAPT WE WERE ONLY CLRED TO FL240; WHILE LOOKING UP AND SEEING FL250 IN THE ALT WINDOW. I SAID 'NO; I ANSWERED CLRNC ONLY TO FL240.' HE SAID; 'BUT THE WINDOW SAYS FL250.' I CONVINCED HIM TO DSND TO FL240 UNTIL WE COULD GET VERIFICATION OF THE ASSIGNED ALT. THE CTLR; WHEN ASKED 'TO VERIFY CLRED TO FL250' SAID 'NO; CLRED ONLY TO FL240 BUT NOW CLRED TO FL250.' WE HAD CLBED TO ABOUT FL245 THEN DSNDED BACK TO FL242 BEFORE BEING CLRED TO FL250. WE COULD NOT FIGURE OUT HOW FL250 GOT INTO THE WINDOW. THIS WAS OUR 5TH LEG FLYING TOGETHER AND WE ALWAYS VERIFIED THE ALT WINDOW SETTING. AN EXPLANATION CAME AS WE MADE OUR APCH INTO AUS. WE WERE ASSIGNED 3000 FT AS WE DSNDED INTO AUS BEING VECTORED TO RWY 13R. AS WE WERE LEVELING AT 3000 FT (PASSING AROUND 3200 FT) WE RECEIVED VISUAL APCH CLRNC; LOOKED UP AND SAW THE ALT WINDOW HAD CHANGED TO 4000 FT BY ITSELF. IT BECAME APPARENT THAT THE ALT ALERT HAD CHANGED BOTH TIMES BY ITSELF. WE ALSO FOUND THAT IT HAD A 'DEAD SPOT' WHERE CHANGING IT 1000 FT CLICK; THE ALT WOULD NOT CHANGE. WE BOTH WROTE UP THE ALT ALERT IN THE LOGBOOK FOR MAINT ACTION TO CORRECT THE ROAMING AND DEAD SPOT.