FLC OF B777 HDG OUT OVER THE ATLANTIC EXPERIENCES A SUDDEN HARD JOLT DURING CLB. DECISION IS MADE TO JETTISON FUEL AND RETURN FOR A MAINT CHK.

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WE EXPERIENCED A ROUTINE FLT DEPARTING EWR TO LHR AND THE 3 OF US HAD FLOWN TOGETHER ALREADY ON 3 PREVIOUS FLTS THIS MONTH. THE NIGHT SKY WAS CLR AS WE CLBED TO OUR CRUISING ALT OF FL350. PASSING THROUGH FL245 JUST E OF HFD VOR; WE EXPERIENCED A SUDDEN JOLT OF THE ACFT. IT WAS LIKE HITTING A HIDDEN POTHOLE AT 60 MPH. JUST 1 JOLT; VERY SUDDEN AND THEN GONE. MY FIRST THOUGHT WAS A GLANCING MIDAIR WITH ANOTHER ACFT ALTHOUGH NONE WERE NEARBY. THAT ALSO RULED OUT WAKE TURB. FACING 6 HRS AT NIGHT OVER OPEN OCEAN; WE RELUCTANTLY DECIDED TO RETURN TO EWR FOR AN INSPECTION. THIS WAS MADE WITH THE KNOWLEDGE OF A RECENT SECURITY THREAT TO EAST COAST AIRLINES REGARDING MIDDLE EAST TERRORISTS. ABOUT 35000 LBS OF FUEL WAS JETTISONED DURING DSCNT ON OUR DIRECT RTE TO EWR. ALTHOUGH WE DIDN'T DECLARE AN EMER; BY ASKING FOR RESCUE EQUIP TO STAND BY; ATC DECLARED IT FOR US. A ROUTINE APCH AND SMOOTH OVERWT LNDG WAS MADE AT EWR. MAINT THOROUGHLY INSPECTED THE ACFT WHILE ALL CARGO WAS OFFLOADED. WHEN NOTHING UNUSUAL WAS FOUND; THE FLT WAS RELOADED AND CONTINUED ON TO LHR 4 HRS LATE.