THE ALT ALERTER TENDS TO 'SKIP' FROM ONE SET ALT TO ANOTHER.

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ON A RECENT FLT; THE ALT ALERTER CHANGED BY ITSELF. WHILE WE WERE CLBING TO 11000 FT; WE WERE RECLRED TO 17000 FT. I CRANKED THE ALT KNOB CLOCKWISE TO 16000 FT AND THEN TURNED IT SLOWLY 1 CLICK. THE WINDOW SHOULD HAVE DISPLAYED 17000 FT BUT IT REMAINED ON 16000 FT. I THEN CLICKED IT SLOWLY 1 MORE CLICK AND IT CHANGED TO THE CORRECT SETTING OF 17000 FT. THE FO VERIFIED THE ALT. I ALSO CHKED MY FMA TO VERIFY 17000 FT WAS ARMED. WE HAD ONLY CLBED A FEW THOUSAND FT WHEN I NOTICED THAT 18000 FT WAS SET IN BOTH THE FMA AND ALT SET WINDOW ON THE DFGP. DURING ALL THIS; WE WERE IN A CONSTANT AIRSPD CLB ON A SET HDG. NEITHER THE FO NOR I TOUCHED ANYTHING ON THE FLT GUIDANCE CTL PANEL. IT IS NOT UNUSUAL; WHEN CHANGING THE COURSE SELECTOR ON THE NAV RECEIVER; TO HAVE TO TURN THE KNOB MORE THAN 1 CLICK TO CHANGE THE SETTING 1 DEG. WHEN THIS HAPPENS; YOU QUITE OFTEN FIND THAT LATER THE COURSE SELECTED HAS CHANGED 1 DEG L OR R. I BELIEVE A SIMILAR THING OCCURRED WHEN IT REQUIRED 2 CLICKS OF THE KNOB TO CHANGE THE ALT 1000 FT. THIS INCORRECT ALT WAS RELATIVELY EASY TO SPOT BECAUSE YOU ARE ALMOST NEVER CLRED TO CLB TO FL180 DURING A CLBOUT TO A HIGH ALT SECTOR. THERE WAS NO ALT BUST IN THIS CASE. THE MD-80 ALT ALERT SYS SEEMS TO BE IN NEED OF A FIX JUDGING FROM ALL THE ASAP RPTS FILED ON ALT BUSTS IN THE ACFT.