FLT CREW OF MD88 ENCOUNTER INEXPLICABLE AND UNRECOVERABLE SPD LOSS AT FL340. RECOVERY REQUIRED DSCNT TO FL320.

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WHILE CRUISING AT FL340; THE ACFT LOST APPROX 60 TO 70 KTS AT A RAPID RATE. THE CONDITIONS WERE VFR WITH NO ICE VISIBLE NOR HAD WE BEEN IN ANY FOR THE PREVIOUS 30 MINS. AS THE EVENT UNFOLDED; THE CAPT AND I WERE DISCUSSING THE SYSTEMS OF THE AIRPLANE AND SPECIFICALLY THE ENGINES WHEN THIS EVENT OCCURRED. WE JUST HAPPENED TO BE LOOKING AT THE ENGINE INSTRUMENTS AT THAT MOMENT. THERE WAS NO ABNORMAL READINGS AT THAT MOMENT. WE FELT A VIBRATION THAT CAUGHT OUR ATTENTION AND LOOKED AT THE AIRSPEED. THAT IS WHEN WE SAW IT HAD FALLEN FROM 280 TO 220 KTS. THE CAPT (HIS LEG) SELECTED CLB ON THE TRP AND EVEN MANUALLY PUSHED THE PWR UP WITH NO HELP WITH THE AIRSPEED. WE INITIALED A DSCNT IMMEDIATELY FROM FL340 TO FL320. THERE WAS NO TFC CONFLICT WITH WASHINGTON CTR AND THERE WAS NOTHING SAID BY THE CTLR. WE ANALYZED THE SIT AS BEST AS WE COULD BUT COULD NOT FIND ANY REASON THE AIRPLANE LOST THE AIRSPEED IT DID. WE CONSULTED WITH MAINT AND CONTINUED ON TO ATLANTA. FOR THE NEXT 1.5 HRS OF FLYING; WE HAD NO PROBS WITH THE AIRPLANE AT ALL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT TO DATE NO EXPLANATION FOR THE INCIDENT HAS BEEN PROVIDED TO HIM. RATHER NEW ON THE AIRPLANE; HE IS UNAWARE BUT DOUBTFUL AS TO WHETHER IT IS EQUIPPED WITH A DIGITAL FLT DATA RECORDER THAT COULD PROVIDE DETAILED INFO REGARDING FLT CONDITIONS AND AIRFRAME AND ENGINE PARAMETERS WHICH MIGHT PROVIDE AN EXPLANATION. THEY CONTINUED TO THEIR DEST AT THE LOWER ALT AND NO FURTHER PERFORMANCE ISSUES WERE ENCOUNTERED.