MD90 FAILS TO AUTO CAPTURE ALT DURING DSCNT TO CVG. FLT CREW AND APCH CTLR MAKE TIMELY CORRECTION.

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WE HAD JUST CROSSED OBBAN ON THE SWEED ARR INTO CVG. WE CROSSED OBBAN AT 11000 FT AND WERE EITHER ENRTE TO FLM OR ALREADY ON A VECTOR; WHEN APCH GAVE US A 'DSND AND MAINTAIN 9000 FT' CLRNC. AS WE APCHED 9000 FT; I NOTICED THE ALT CAPTURE DID NOT CAPTURE (LOST LOCK) AS WE WERE APPROX 100-150 FT BELOW 9000 FT AND STILL IN A SLIGHT DSCNT. THE AUTOPLT WAS DISCONNECTED AND AN IMMEDIATE CLB BACK TO 9000 FT WAS INITIATED. THE TIME DELAY OF RECOGNIZING THE ALT HAD NOT CAPTURED AND MAKING SURE THE ALT REVERSAL DIDN'T HURT ANY PAX; OUR ALT DIPPED 280 FT BELOW 9000 FT. APCH CTL QUESTIONED OUR ALT AS WE HAD REVERSED THE DSCNT INTO A CLB. THERE WERE NO TFC CONFLICTS AND NO ONE ON OUR TCASII THAT WE COULD SEE. WE ACKNOWLEDGED THAT WE WERE CLBING BACK TO 9000 FT WITH CVG APCH AND THE CTLR DIDN'T SEEM UPSET WITH US. THE MD88/90 FLT GUIDANCE PANEL DOES HAVE THIS GLITCH AT TIMES OF NOT 'CAPTURING' THE SET ALT. SOMETIMES IT IS OPERATOR ERROR IN THAT ADJUSTING THE 'VERT SPD' AS THE ALT IS CAPTURING WILL CAUSE THE LOSS OF ALT CAPTURE; BUT THERE ARE TIMES WHERE THE 'ALT CAPTURE' WILL LOSE LOCK FOR NO REASON AS IT IS CAPTURING. I'VE SEEN THIS HAPPEN 6 OR 7 TIMES IN ALMOST 7 YRS ON THE PLANE; AND JUST LUCKILY CAUGHT THE 'LOSS OF ALT CAPTURES' IN TIME. WE APPRECIATED APCH CTL BACKING US UP AS SOON AS THEY SAW US STARTING TO GO THROUGH OUR ALT. THEIR QUICK RELAY TO US WAS GREAT BACKUP AND COULD BE INVALUABLE IN PREVENTING A CONFLICT.