HDG TRACK DEV DURING AN AMENDED CLRNC RTE CHANGE. ARTCC RADAR CTLR INTERVENTION.

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THE FLT WAS GIVEN A CLRNC CHANGE WHILE ENRTE. WE WERE IN THE VICINITY OF THE CROSS CITY VOR WHEN ZJX CLRED US TO PROCEED DIRECT JOHN INTXN; J-91 ATL; DIRECT LOUISVILLE; IN. WE WERE USING OMEGA NAV SYS AS THE PRIMARY NAV SYS AND PROGRAMED THE CHANGE. WHEN WE CROSSED JOHN INTXN THE OMEGA NAV SYS BEGAN TO 'DRIFT' AND WE PARALLELED J-91 INSTEAD OF INTERCEPTING. NEITHER THE CAPT NOR MYSELF NOTICED THE 'DRIFT' UNTIL APPROX 40-50 NM SE OF ATL. THE ATL CTLR CALLED AND ASKED WHY WE WERE NOT NAVING J-91. WE THEN EXPLAINED WE WERE HAVING OMEGA NAV SYS PROBS AND ATL RECLRED US DIRECT ATL. WE ASKED IF WE SHOULD CONTACT THEM UPON ARRIVING AT IND AND THEY STATED NOT TO WORRY ABOUT IT; THAT IT WAS NO PROB. WE NAVIGATED THE REST OF THE FLT USING VOR AS THE PRIMARY NAV SYS AND THE FLT WAS UNEVENTFUL. I FEEL THAT THIS COULD HAVE BEEN AVOIDED HAD WE BEEN MONITORING THE OMEGA NAV SYS ACCURACY BY UTILIZING THE VOR SYS AS A BACKUP. THIS PROC IS OUTLINED IN OUR OPS MANUAL AND THERE IS NO SUBSTITUTION FOR GOOD CROSS MONITORING. WE HAD FLOWN 3 LEGS PRIOR TO THIS ONE AND THE OMEGA NAV SYS HAD BEEN WORKING WITHIN LIMITATIONS. I FEEL THAT THIS LED US TO BECOME COMPLACENT IN OUR DUTIES WHICH WOULD HAVE LED TO EARLY DETECTION OF A PROB. ALL 3 CREW MEMBERS LEARNED A VALUABLE LESSON IN COCKPIT RESOURCE MGMNT AND THAT PROCS ARE DEVELOPED TO KEEP US ON COURSE; AND NOT JUST TO KEEP US BUSY. I HAVE LEARNED TO KEEP A GOOD XCHK OF ALL NAV SYS. EARLY DETECTION IS THE KEY.