FLC OF A WDB LOST CTL OF THE ACFT WHEN THE ACFT SUDDENLY STARTED CLBING WITH AIRSPD RISING DURING HIGH ALT CRUISE FLT BTWN TSTM BUILDUPS.

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THE ACFT WAS IN CRUISE AT FL390. THE RADAR SHOWED WHAT APPEARED TO BE A CLR PATH THROUGH A LINE OF TSTMS. THE STORM WAS DEVELOPING ON THE L AND R SIDE OF THE ACFT FLT PATH. WE WERE ON TOP OF ALL THE WX AND COULD SEE THE STARS ALL THE WAY DOWN TO THE HORIZON. THE ACFT WENT FROM SMOOTH FLT TO MACH BUFFETING INSTANTLY. THE CAPT TURNED OFF THE AUTOPLT AND ALLOWED THE ACFT TO PITCH UP AS I PULLED THE THROTTLES BACK TO 65 PERCENT. THE ALT CLBED TO APPROX 40000 FT; WHILE THE ACFT WAS STILL IN MACH BUFFET. THE AIRSPD DROPPED 60 KTS JUST AS QUICKLY AS IT HIT MACH BUFFET. I YELLED; 'AIRSPD' JUST AS THE CAPT PUSHED THE NOSE OVER AND I ADDED PWR TO GAR THRUST. THE AIRSPD RECOVERED AT ABOUT FL365; AS I RECALL; ALTHOUGH THE CAPT THOUGHT IT WAS CLOSER TO FL380. THE 1000+ FT DROP SENT SOME OF THE FLT ATTENDANTS AND MAINT PERSONNEL IN THE REAR OF THE ACFT TO THE CEILING AND HELD THEM THERE FOR 10-15 SECONDS. NO ONE BROKE ANY BONES; BUT THE CAUSE OF ANY BRUISES AND A GREAT DEAL OF ANXIETY CAN BE LARGELY ATTRIBUTED TO THE FACT THAT; ALTHOUGH THE SEAT BELT SIGN WAS ON; THE EXPERIENCED CABIN CREW IGNORED THE SIGN AND HAD NOT STOWED MUCH OF THEIR LUGGAGE. THE FLT WAS OPERATING AS A FAR PART 91 FERRY BACK TO OUR HOME BASE. THEREFORE; IT APPEARS SOME OF THE CABIN CREW DID NOT TAKE THE FACT THAT THE CAPT HAD THE SEAT BELT SIGN ON SERIOUSLY. ALSO; I GOT THE IMPRESSION THAT THE FEAR IN SOME CABIN CREW MEMBERS GENERATED MORE FEAR IN OTHER CABIN CREW MEMBERS. I WILL NEVER KNOW FOR SURE IF THE GAP BTWN STORM CELLS CLOSED TOO QUICKLY; THE WX DEVELOPED RAPIDLY FROM BELOW; OR IF THE ANVIL OF THE STORM CELL ON THE L WAS PUSHED INTO OUR FLT PATH BY THE 130 KT JETSTREAM. I BELIEVE THE LATER TO BE THE CASE; SINCE WE COULD HEAR ICE PELLETS ON THE WINDSHIELD DURING MOST OF THE TURB. IN THE FUTURE I WILL GIVE A STORM CELL MORE DISTANCE AND MAKE THE CABIN CREW BUCKLE UP.