AN MD80 CREW RPTS THE FMS DROPPED A WAYPOINT CAUSING THEM TO DEV FROM THEIR FLT PLAN.

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FO AND MYSELF VERIFIED ROUTING TO ORD... CTR CAM SYR J63 EHMAN YXU...ORD. IN CLB OUT BOS CTR CLRED US DIRECT TO CTR. I WAS HAND FLYING AND FO PUT CTR IN THE FMS. WE BOTH CONCURRED IT LOOKED GOOD. NEXT CTLR ASSIGNED US DIRECT YXU OUT OF 20000 FT ON A CLRNC TO 21000 FT. AT APPROX 16000 FT; BOS TURNED US 30 DEGS TO THE R AND ASKED WHERE WE WERE GOING. SHE THEN SAID WE WERE NOT CLRED TO YXU UNTIL 20000 FT. MY FO EXPLAINED TO HER WE WERE STILL ON FLT PLAN AND HAD NOT SELECTED YXU; HE THEN PROCEEDED TO TELL HER WE WERE ON FLT PLAN TO SYR. SHE REPLIED WITH WE SHOULD BE HEADING TO CAM. WITH NO ALTERATION TO OUR ORIGINAL ROUTING AND THE NUMBER OF ITEMS WE WERE BUSY WITH AS THE ACFT TURNED TOWARDS SYR WE BOTH ASSUMED WE HAD PASSED CAM. AFTER PUTTING CTR IN THE FMS (FO) WE REALIZED IT HAD DROPPED OUT. UPON LNDG IN ORD; I PHONED A FELLOW CAPT; WHO WAS FAMILIAR WITH OUR EVENT AS IT HAD HAPPENED TO HIM (EXACTLY THE SAME WAY AND ON THE SAME FLT AND SEQUENCE). ATC CLRED US ON OUR WAY AT 21000 FT TO YXU AND SAID NOT TO WORRY ABOUT IT. WE ARE SUBMITTING A RPT ANYWAY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT STATES THAT HE HAS OVER 2 YEARS EXPERIENCE ON THE MD80 AND COULD NOT RECALL HAVING A WAYPOINT DROP OUT PREVIOUSLY. HE KNOWS OF OTHER PLTS WHO HAVE HAD PROBS; THOUGH.