AN A320 DEVELOPED A MODERATE AIRFRAME BUFFET AT 320 KTS/M.78/FL270. BUFFET CEASED AFTER CREW SLOWED TO M.71/FL310 AND CREW CONTINUED TO DEST.

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CLBING THROUGH FL270 AT M 0.78 320 KTS. WE ENCOUNTERED LIGHT CHOP. THE AIRFRAME STARTED TO VIBRATE FROM HORIZONTAL STAB FLUTTER. THE FLUTTER WAS MODERATE AND CAUSED US TO QUESTION THE INTEGRITY OF THE TAIL ASSEMBLY. UPON PULLING BACK AND SLOWING; THE FLUTTER DISAPPEARED. WE KEPT THE SPD SLOW FOR THE REMAINDER OF THE FLT. THE AIRPLANE PERFORMED NORMALLY AS SOON AS WE SLOWED DOWN; WITH NO INDICATIONS OF FURTHER PROBLEMS. AS DEN HAD A WX SYSTEM MOVING IN; AND ALREADY HAD VIRGA AT DEP TIME; WE ELECTED TO CONTINUE ON AS THE BALANCE OF THE FLT HAD CLR SKIES AND RELATIVELY SMOOTH CONDITIONS. THE CLB OUT OF DEN WAS QUITE TURBULENT; AND WE DID NOT WANT TO GO BACK INTO THAT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE UNCOMMANDED MOVEMENT WAS OF SUFFICIENT INTENSITY THAT HE DIRECTED THE FO (PF) TO DISCONNECT AUTOFLT AND HAND FLY THE ACFT WHILE SLOWING. IN DISCUSSING THE EVENT WITH MAINT AND FOLLOWING THE ACFT'S MAINT HISTORY FOR SEVERAL DAYS AFTER THIS EVENT; HE DISCOVERED THAT AN ELEVATOR SERVO VALVE WAS MALFUNCTIONING. HE FURTHER DISCUSSED THE EVENT WITH A MECHANIC AND WAS TOLD THAT THE SERVO MALFUNCTIONS WERE BECOMING COMMON FLEET WIDE.