A B737-300 OVER FST DEVIATED 600 FT FROM ASSIGNED ALT BECAUSE OF TURB NEAR TSTMS.

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A LARGE LINE OF TSTMS WAS RUNNING APPROX N AND S IN WEST TEXAS ACROSS OUR PROPOSED RTE OF FLT. OUR CLRNC WAS DIRECT TO SSO AND WE ENTERED IMC. ONE PARTICULARLY LARGE TSTM CAUSED US TO ASK FOR AND RECEIVE CLRNC TO DEVIATE S OF OUR COURSE BY ABOUT 30 DEGS. AS WE DEVIATED AND THE RADAR SWEEP STABILIZED 3 VERY SMALL SEPARATE RED AREAS OF CONVECTIVE ACTIVITY APPEARED ON OUR CHOSEN TRACK. I VERBALIZED THAT I WOULD HAVE PREFERRED TO HAVE TURNED HARD R AND GONE BTWN THE LARGE STORM AND THE 3 VERY SMALL ONES; BUT I DIDN'T THINK ACFT PERFORMANCE WOULD ALLOW. I TURNED L IN AN ATTEMPT TO ROUND THE VERY SOUTHERN END OF THE 3 SMALL CELLS. IN THIS ATTEMPT MODERATE TURB WAS ENCOUNTERED FOR ABOUT 3 SECONDS; BUT UP- AND DOWNDRAFTS CAUSED THE ACFT TO GAIN ABOUT 300 FT AND THEN LOSE ABOUT 600 FT ALL WITH THE AUTOPLT ENGAGED IN ALT HOLD MODE. I ATTEMPTED TO ACCELERATE THE RETURN TO THE ASSIGNED ALT BY OVERRIDING THE AUTOPLT IN NOSE UP PITCH; BUT IT CORRECTED AS I MADE MY INPUT. FO MADE AN IMMEDIATE CALL TO ATC TO RPT THE RIDE AND THE ALT EXCURSION. NO INJURIES. MY MAJOR CONCERN IS THAT WE WERE IN RVSM AIRSPACE AND BECAUSE OF NATURAL FORCES; WERE OFF OUR ASSIGNED ALT. WHEN CONVECTIVE WX DICTATES OPPOSITE DIRECTION ACFT GO THROUGH THE SAME 'HOLES;' RVSM SHOULD BE SUSPENDED AND 2000 FT VERT SEPARATION RE-INSTITUTED.