AN ACR WDB HAD AN INADVERTENT SLAT DEPLOYMENT WITH A LOSS OF ALT.

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I AM A CHK AIRMAN FOR AN ACR. I WAS RETURNING TO JFK FROM LHR WITH A NEW CAPT IN THE L SEAT. I NOTICED THAT THE FLAP/SLAT HANDLE AND THE SLAT STOW LEVER SEEMED 'NOT QUITE RIGHT' IN APPEARANCE. TO ASSURE PROPER STOWAGE OF THE FLAP/SLAT HANDLE; I REACHED OVER AND CYCLED THE SLAT STOW LEVER (AS PER PROC) TO ASSURE PROPER STOWAGE OF THE FLAP/SLAT HANDLE. THE INSTANT I PUSHED THE SLAT STOW LEVER FORWARD; THE SLATS DEPLOYED AND BUFFETING OCCURRED. I DISCONNECTED THE AUTOPLT; LEVELED THE WINGS; AND ADDED PWR. WE LOST 1000 FT BUT REGAINED CTL WHEN I PUSHED THE FLAP/SLAT HANDLE FORWARD TO SEAT IT. WE CLBED BACK TO FL350 AND TOLD GANDER EVERYTHING WAS OK. I NOTICED THAT; UPON RETRACTING THE SLATS ON TKOF AT LHR; THE SLAT STOW LEVER FELT 'ODD' BUT WAS SATISFIED OF ITS OP AT THAT TIME. I FORGOT ABOUT THE SLAT STOW LEVER. EVERYTHING ABOUT THE FLT SEEMED NORMAL. WHEN WE WERE IN CRUISE AT FL350/.83 MACH AND IN A RELATIVELY RELAXED ATMOSPHERE (ABOUT 1 HR FROM TOUCHDOWN) DID I BECOME AWARE OF THE FLAP/SLAT HANDLE AND SLAT STOW LEVER APPEARANCE. AS WE WERE LOSING ALT; GANDER CALLED AND ASKED IF WE WERE OK. WE RESPONDED THAT WE HAD HAD A PROB BUT EVERYTHING NOW WAS UNDER CTL. I SUSPECT THE WHOLE THING LASTED ABOUT 30 SECONDS (SEEMED LIKE A LIFETIME OF BUFFETING). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING CAPT STATES THAT HE LOST 1000 FT IN THIS MANEUVER AND THAT CZQX IMMEDIATELY NOTED THIS AND OFFERED ASSISTANCE. THE 2 POS SLATS HAVE A 16 SECOND CYCLE FROM UP TO DOWN AND BACK UP. THE ACFT BUFFETED WILDLY DURING THIS TIME. THE RPTR TOOK THE ACFT OFF OF THE AUTOPLT DURING THE GYRATIONS. THE ACFT MANUFACTURER HAS PUT IN A TEMPORARY FIX CONSISTING OF A LATCH THAT HOLDS THE FLAP/SLAT HANDLE IN THE UP/RETRACT POS. IT REQUIRES 2 HANDS TO MOVE THE HANDLE TO THE SLAT EXTEND POS. A PERMANENT FIX IS IN THE WORKS THAT WILL REPLACE THE CURRENT MECHANICAL MONSTROSITY WITH AN ELECTRICAL SWITCH. THE RPTR STATES THAT HE FLEW A TEST HOP ON THIS ACFT THE NEXT DAY AND THE SYS WORKED PROPERLY. MAINT FOUND THAT THE HANDLE WAS NOT PROPERLY RIGGED; THAT THERE WAS TOO MUCH TENSION ON THE FLAP/SLAT HANDLE.