FLT CREW OF F2TH DEVIATE FROM CLRED TRACK DUE TO FMC IRREGULARITY. ENSUING CONFUSION RESULTS IN DEV FROM ASSIGNED SPD.

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DURING CLBOUT; DEP CTL CLRED US DIRECT TO LDN; CLB TO 17000 FT AND TO MAINTAIN 250 KIAS. THE FMS WAS PROGRAMMED FOR DIRECT LDN. THE ACFT STARTED A L TURN. THE CREW REALIZED THIS WAS THE WRONG DIRECTION AND TOOK CORRECTIVE ACTION. THE FMS SHOWED A COURSE TO LDN AS 070 DEGS AND APPROX 256 NM. THE CORRECT COURSE WAS APPROX 270 DEGS AND 15 NM. WHILE TURNING BACK TO THE R DIRECT LDN; ATC QUERIED US AS TO OUR DIRECTION OF FLT. THE CREW EXPLAINED THAT THE FMS WAS 'CONFUSED' AND WE WERE TURNING BACK TO THE R DIRECT LDN. WHILE THE CREW WAS TRYING TO DETERMINE WHAT WAS WRONG WITH THE FMS AND ITS POS; THE ACFT WAS LEVELED AT 17000 FT. WHILE THE CREW'S ATTN WAS DRAWN TO THE FMS; THE ACFT SPD INCREASED TO AROUND 320 KIAS. ATC CALLED AND INSTRUCTED US TO MAINTAIN SPD OF 250 KIAS. THE CREW PROMPTLY COMPLIED. THERE WERE NO KNOWN TFC CONFLICTS; NO TCASII RA'S; AND ATC DID NOT ADVISE THAT ANY PROBS WERE CAUSED. THE PROB WAS CAUSED BY THE CREW BEING DISTR BY THE 'LOST' FMS IN A BUSY TERMINAL AREA. THE CREW SHOULD HAVE PROMPTLY ADVISED ATC THAT THERE WAS A PROB WITH THE NAV SYS AND REQUESTED ASSISTANCE FROM ATC UNTIL THE SIT WAS RECTIFIED.