ALTDEV ALT EXCURSION -- DURING A TURN IN THE HOLDING PATTERN THE AUTOFLT SYS ALLOWED THE ACFT TO CLB; LOSING AIRSPD. THE FLC STARTED A DSCNT TO GAIN AIRSPD; BUT WENT BELOW ASSIGNED ALT.

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WHILE FLYING AT FL370 WE WERE INSTRUCTED TO HOLD AT AN INTXN ABOUT 50 MI AHEAD OF US APPROX HALF WAY BTWN DTW AND JHW. FO SLOWED TO CHARTED HOLD SPD OF 237 KTS AND ENTERED HOLD. ON FIRST 180 DEG TURN; OUTBOUND; THE ACFT TRIED TO CLB; AND FO CORRECTED AT APPROX 150 FT HIGH. THE AIRSPD LOSS WAS ABOUT 5 KTS; AND WHILE WORKING ON FUEL COMPUTATION WENT UNNOTICED. NEXT TURN; INBOUND; 10 MI LEG ACFT AGAIN ATTEMPTED TO CLB AND FO AGAIN CORRECTED BACK TO FL370. THE SUBSEQUENT AIRSPD LOSS WAS NOT REGAINED AS PWR WAS ALREADY AT MAX. AT APPROX 90 DEGS OF TURN WE EXPERIENCED ONSET OF AIRFRAME STALL BUFFET. NO STICK SHAKER ACTIVATED; AND NO AUTO-SLAT EXTENSION. WE HAD TO MAKE THE INBOUND TRACK GOOD AS THERE WERE ACFT ON THE OTHER SIDE OF THE HOLDING PATTERN. WHILE TRYING TO DETERMINE THE PROB WE BEGAN A 300-400 FT RATE OF DSCNT TO MAINTAIN AIRSPD. STALL BUFFET OCCURRED 3-4 TIMES AND WE CONTINUED OUR DSCNT. AT APPROX FL367 ATC ASKED IF WE WERE AT FL370. WE REPLIED NEGATIVE; AND THAT WE COULD NOT MAINTAIN FL370. AFTER SOME CONVERSATION ATC ASKED IF WE WOULD BE MORE COMFORTABLE AT FL350. WE SAID YES; AND WE WERE CLRED TO FL350. FO ACCELERATED TO 250 KTS IN THE DSCNT; AND WE EXPERIENCED NO FURTHER PROBS AT FL350.