FLC OF DC9 EXPERIENCES A RAPID DECREASE IN MACH SPD AND THEN SEVERAL GOOD JOLTS OF TURB CAUSING AN ALTDEV; BOTH UP AND DOWN.

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CRUISING LEVEL AT FL310. CAPT AND I NOTICED MACH INDICATION DECREASED RAPIDLY FROM .79-.80 TO .74-.75. WE NOTICED A FEW CLOUDS THAT MIGHT INDICATE POSSIBLE TURB. 'LIGHT' HAD BEEN RPTED IN THE AREA S. I PUT ON THE 'FASTEN SEAT BELT' SIGN AND MADE A SAFETY ANNOUNCEMENT. WE ENCOUNTERED 1-2 GOOD JOLTS; BUT JUST LIGHT TO MODERATE TURB (MOSTLY SMOOTH TO LIGHT). HOWEVER; THE MACH RAPIDLY INCREASED BACK TOWARD .80 AND REACHED .82 MACH BRIEFLY. WE HAD REQUESTED FL280 WHEN THE AUTOPLT BEGAN DSNDING THE ACFT FROM FL310; WITH NO ACTION FROM THE CAPT. I SAID 'CHK ALT.' AT APPROX FL307 THE ACFT BEGAN TO CLB BACK UP TOWARD FL310 AND REACHED APPROX FL314. OUR INITIAL DSCNT DID CAUSE OUR TCASII TO GIVE AN RA FOR THE ACFT 2000 FT BELOW US. TCASII INDICATED THE ACFT 1900 FT BELOW US. WE BOTH BELIEVE THAT THERE MUST HAVE BEEN 'WAVE ACTION' SIMILAR TO MOUNTAIN WAVES NEAR OUR ALT AND POS. FOR SURE THE ATMOSPHERIC CONDITIONS HAD CHANGED QUICKLY AND THE DC9 AUTOPLT ATTEMPTED CORRECTION. WE HAD NO OTHER PROBS THAT MIGHT INDICATE AUTOPLT MALFUNCTION. I FEEL THAT UNUSUAL UPDRAFT/DOWNDRAFT WAVE TYPE ACTION MAY HAVE BEEN THE REASON FOR THE AUTOPLT'S SUDDEN DEV FROM ALT. THE DEVS WERE APPROX 300-400 FT FROM FL310. THE CAPT; AS PER SOP FOR OUR COMPANY; ALLOWED THE AUTOPLT TO CORRECT. JUST AS HE WAS ABOUT TO DISENGAGE AND RETURN TO FL310; THE AUTOPLT BEGAN TO CLB THE ACFT. AGAIN; THE DEVS IN AIRSPD AND ALT WERE SUDDEN. ALTDEV WAS BRIEF. THE CAPT AND I BELIEVE OUR ACTIONS WERE PROMPT AND DELIBERATE AND VERY CONCERNED WITH THE SAFE OP OF THE ACFT. NO ONE INJURED.