FLC OF AN MD88 WERE TAKEN OFF COURSE BY AN ERROR IN THE FMS. ARTCC RADAR CTLR INTERVENED AND VECTORED THEM ON A DIRECT COURSE TO THE NEXT VOR.

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DURING OUR CLB TO CRUISE ALT; THE WIND VECTOR COMPUTED BY THE FMC SHOWED THE WIND TO BE APPROX 295 DEGS AT 70 KTS. THE AUTOPLT AS WELL AS THE FMS LATERAL NAV MODE WERE ENGAGED. THE ACFT APPEARED TO BE TRACKING THE COURSE CORRECTLY BASED ON THE DISPLAYED WIND INFO. IN THE VICINITY OF LANCASTER VOR; THE DISPLAYED WIND VECTOR ABRUPTLY SHIFTED TO INDICATE WINDS FROM THE SSW AT 160 KTS. WE NOTICED THE SHIFT; BUT WERE NOT UNDULY CONCERNED BECAUSE WE WERE EXPECTING STRONG WINDS AT ALT (WE WERE FLT PLANNED FOR A LOWER CRUISE ALT THAN NORMAL DUE TO FORECAST STRONG WINDS ALOFT). THE AUTOPLT CHANGED ACFT HDG APPROX 30 DEGS L IN RESPONSE TO FMC COMMANDS (REFLECTING THE NEW WIND INFO). WITH THE AUTOPLT FLYING THE NEW HDG; THE FMS/EFIS SHOWED US TRACKING THE COURSE CORRECTLY (J6 FROM LRP TO MRB) ALBEIT WITH A 20-30 DEG L WIND CORRECTION ANGLE. AFTER SOME TIME ON THIS HDG; ATC QUERIED US ABOUT OUR COURSE; GAVE US A HDG FOR DIRECT MRB VOR AND ASKED US TO HAVE OUR NAV EQUIP CHKED. FOR THE REMAINDER OF THE FLT WE MADE SURE THE PNF DISPLAYED RAW DATA ON HIS EFIS TO XCHK FMC OP. WE DID NOT NOTICE ANY ADDITIONAL PROBS ON THAT FLT. WE ENTERED THE FAULT IN THE ACFT MAINT LOGBOOK AND INFORMED FLT CTL OF THE SIT VIA ACARS. IT IS NOTEWORTHY THAT THE ERRONEOUS HDG COMMANDED BY THE FMC WOULD HAVE BEEN CORRECT IF THE WIND REALLY WAS SSW 160 KTS. THE PROB; THEREFORE; SEEMS TO HAVE ARISEN BECAUSE FOR SOME REASON THE FMC COMPUTED INCORRECT WIND INFO. IN FUTURE; ANY ABRUPT CHANGE IN LATERAL/VERT NAV BY THE AUTOPLT/FMS SHOULD BE IMMEDIATELY INVESTIGATED IN ORDER TO AVOID GROSS NAV ALT ERRORS.