A GLASS COCKPIT ACR CREW HAD A MAP SHIFT THAT SHOWED THE ACFT ABOUT 90 MI FROM ITS VOR POS.

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DURING CRUISE FLT BTWN KLGA AND KPIT; WE RECEIVED A MESSAGE ON BOTH FMC'S THAT WE'RE RECEIVING IRS-ONLY NAV; WHICH MEANS THEY (FMC'S) WERE NOT XCHKING THE NAV WITH VOR AND DME. SWITCHING TO MANUAL VOR AND DME NAV; WE REALIZED BOTH EFIS MAP DISPLAYS WERE OFF APPROX 90 MI. WE PROCEEDED WITH MANUAL VOR AND DME NAV WITH NO CONFLICT. CTR HAD VECTORED AND GIVEN DIRECT ROUTING MOST OF THE 50 MIN FLT; SO I DON'T THINK THEY SUSPECTED OUR NAV WAS OFF. THE ALL 'GLASS' COCKPIT ACFT REQUIRE CONSTANT MONITORING AND AT TIMES CAN BE MORE CONFUSING THAN CONVENTIONAL ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR DESCRIBED THIS PROBLEM AS 'MORNING SICKNESS.' THIS ACFT; AND OTHER GLASS COCKPIT ACFT HAVE PROBLEMS THAT TEND TO FIX THEMSELVES THROUGHOUT THE DAY; AND MAY NEVER RECUR AGAIN. THE RPTR SAID THAT THE ACFT WAS ON THE GND AT PIT; BUT THAT THE MAP SHOWED THE ACFT TO BE IN THE VICINITY OF SLT; ABOUT 90 NM TO THE NE. HIS COMPANY MAINT GAVE THE SYS A 'FULL ALIGNMENT' AND SENT IT ON ITS WAY WITH NO KNOW FURTHER PROBLEMS.