MD83 CREW HAD AN ALTDEV; AFTER THE AUTOTHROTTLE SYS STARTED TO CLOSE THE ENG THRUST LEVERS. THIS WAS A REPEAT DISCREPANCY.

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UNCOMMANDED AIRSPD REDUCTION A LEVELOFF. THERE WAS A PREVIOUS WRITE-UP IN THE MAINT LOG ABOUT THE ACFT NOT FOLLOWING SPECIFIED PROGRAMMED AIRSPD INPUTS THAT WAS CHKED OUT AND SIGNED OFF. AIRSPD/MACH WERE PROPERLY SET DURING PREFLT AND THUMBWHEEL AND AIRSPD/MACH BUTTONS USED DURING CLB TO ALT. RIGHT AT LEVELOFF FROM A CRUISE CLB; THE THROTTLES CAME BACK ALMOST TO IDLE TRYING TO FOLLOW AN UNCOMMANDED 230 KIAS WITH THE AIRSPD QUICKLY BLEEDING OFF TO ABOUT .72 MACH OR A LITTLE LESS; AND THE ACFT STARTED A DSCNT TO KEEP THE AIRSPD. BOTH AUTOPLT AND AUTOTHROTTLES WERE CLICKED OFF AND A MANUAL RECOVERY INITIATED BACK TO ALT BUT NOT BEFORE LOSING ABOUT 500 FT WITH A QUERY FROM CTR. THE ACFT WAS RESET FOR CRUISE AND WORKED FINE FOR THE REMAINDER OF THE FLT BUT AGAIN WRITTEN UP. THE ONLY SPDS USED TO THAT POINT WERE THE STANDARD 250 KTS TO 10000 FT; 320 KTS TO MACH AND A .75 MACH TO CRUISE WITH NO OTHER INPUTS. THE LOSER AIRSPD IS A MYSTERY TO BOTH OF US. SUPPLEMENTAL INFO FROM ACN 634821: THE AUTOPLT WAS TRANSITIONING FROM CLB TO ALT CAPTURE. AS SOON AS THE FMA WENT TO ALT CAPTURE. AS SOON AS THE FMA WENT TO ALT CAPTURE; I NOTICED THE NAV WINDOW SHOWED NAV HOLD INSTEAD OF NAV TRACK. WE HAD BEEN TRACKING THE FMC COURSE. I THEN LOOKED AT THE AIRSPD INDICATOR AND THE SPD WAS 280 KTS .756 MACH. THE AIRSPD BUG HOWEVER HAD SHIFTED TO 230 KTS. WHEN LEVELOFF OCCURS THE AIRSPD COMMANDED BY THE BUG IS WHAT THE PWR GOES TO. SO 100 FT BELOW 310 KTS THE PWR BEGAN TO RETARD TOWARD IDLE. I CALLED OUT 200 FT LOW AND THE CAPT DISENGAGED THE AUTOPLT AND PUSHED UP THE PWR SMOOTHLY. WE WROTE UP THE AIRPLANE AND THIS WAS THE 3RD SUCH WRITE-UP IN RECENT HISTORY.