WHEN CLBING THROUGH FL394 IN A B737-700; THE ACFT AUTOPLT DISCONNECTED AND BUFFETED 2 TIMES BEFORE THE FLYING FO COULD DSND AND REGAIN COMPLETE ACFT CTL.

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THIS WAS THE SECOND ETOPS FLT FOR THE FO AND MYSELF; HAVING MADE THE FIRST FLT THE DAY BEFORE. THE CHK AIRMAN SITTING IN THE JUMP SEAT WAS FAIRLY NEW AT IT TOO. THIS WAS A NEW AIRPLANE FOR ALL OF US. IT WAS THE FO'S LEG. I HAD FLOWN OVER TO THE DEST THE DAY BEFORE. WE WERE DOING A SLOW RATE OF CLB TO FL400 AS INSTRUCTED BY THE CHK AIRMAN. I WAS GLANCING DOWN WORKING ON THE FLT LOG FOR A COUPLE OF MINS. AS WE CLBED TO ABOUT FL394; THE AUTOPLT DISENGAGED. IT SURPRISED ALL OF US. THE FO SAID; 'I HAVE IT;' AND APPLIED BACK PRESSURE ON THE CTL YOKE WHICH RESULTED IN 2 VERY SLIGHT 'G' BUFFETS. I SAID 'EASY' AND PUT MY HANDS AS THOUGH I WERE BLOCKING ANY MORE AFT MOVEMENT OF THE CTL COLUMN. I THEN CALLED CTR TO DSND TO FL380 AND WE DID. AT THE TIME THE AUTOPLT DISENGAGED; THE PFD AIRSPD DISPLAY AIRSPD WAS INDICATING ON THE TOP OF THE MINIMUM MANEUVERING SPD YELLOW BAR. ALSO DURING THE LAST 5-10 MINS; WE HAD BEEN ENCOUNTERING OCCASIONAL MODERATE TURB. THE 'G' BUFFETS WERE SMALL BUT WE WONDERED IF ANYBODY IN THE BACK FELT THEM. I THINK ALL 3 OF US LEARNED SOMETHING TODAY ABOUT MAX ALT AND TURB COMBINATIONS CAN REDUCE MANEUVERING SPD VERY FAST.