A B767-300 OVER DAG AT FL310 WAS UNABLE TO MAINTAIN RVSM ALT SEPARATION BECAUSE OF MODERATE TURB. ATC ASSIGNED A LOWER ALT.

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AT FL310 OVER DAG; WE ENCOUNTERED MODERATE TURB WITH UP- AND DOWNDRAFTS WHICH CAUSED PITCH AND ROLL OSCILLATIONS WITH ACCOMPANYING ERRATIC PWR FLUCTUATIONS. THE ACFT BEGAN DSNDING AND EVEN AFTER MANUAL INTERVENTION WE WERE UNABLE TO HOLD ALT. DSNDING THROUGH FL305; CAPT ADVISED ATC THAT WE WERE 'UNABLE TO MAINTAIN RVSM.' AT APPROX FL303 (700 FT LOW); ATC GAVE US A DSCNT TO FL270. TURB DECREASED AS WE DSNDED AND THE REMAINDER OF THE FLT WAS UNEVENTFUL. WE WERE OPERATING WELL BELOW OUR OPTIMUM ALT OF FL370 AT THE TIME. SEVERAL OTHER ACFT WERE TALKING TO ATC ABOUT THE TURB BUT WERE TOLD THAT ALL ALTS WERE BAD. THE CAPT HAD THE FLT ATTENDANTS TAKE THEIR SEATS AND CARTS WERE STOWED PRIOR TO US ENCOUNTERING THIS AREA OF CONCENTRATED TURB. WE WERE PREPARED FOR THE TURB IN ADVANCE; OPERATING AT TURB PENETRATION SPD WITH PLENTY OF MARGIN FOR BUFFET; FLT ATTENDANTS SEATED; ETC. WHAT I COULD HAVE DONE BETTER AS THE PF WAS TO BE QUICKER TO TOGGLE OFF THE AUTOPLT AND MORE AGGRESSIVE IN PUSHING THE PWR UP IN AN ATTEMPT TO MINIMIZE ALT LOST. I DID NOT EXPECT THE DOWNDRAFT TO LAST SO LONG AND FIGURED THAT I WOULD MAKE MATTERS WORSE BY OVERDOING THINGS IN THE NEXT UPDRAFT. IN HINDSIGHT; I WOULD BE MORE PROACTIVE; CONSIDERING THE REDUCED SEPARATION OF TODAY'S RVSM ENVIRONMENT. BY THE WAY; THERE WERE NO OTHER ACFT IN OUR VICINITY AT THE TIME.