MD80 CREW WAS REQUIRED TO MAKE A PRECAUTIONARY LNDG; SHORT OF DEST; AFTER FLT INST PROBS SURFACED IN THE ACFT.

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DIVERT DUE TO DEGRADED HORIZON AND COMPASS SYS. DEP FROM THE GATE WENT ACCORDING TO SCHEDULE. ATC DELAY OF NEARLY 2 HRS (HOLDING ON TXWY). NORMAL DEP FOR LGA. AFTER PASSING RDU; WE WERE TURNED BACK TO RDU FOR HOLDING DUE TO ARPT STOPPING ARRS IN THE NEW YORK AREA. UPON ENTERING HOLDING AT RDU (CLR AND DARK CONDITIONS) DURING FIRST TURN HORIZON LIGHT ILLUMINATED. ON ROLLOUT; LIGHT EXTINGUISHED. ON SECOND TURN; HORIZON LIGHT AGAIN CAME ON. IT WAS NOTED APPROX 4 DEGS OF SEPARATION BTWN CAPT'S AND FO'S HORIZON IN THE TURN. ON THIRD TURN; THE HORIZON AND HDG LIGHTS CAME ON. THE HDG DIFFERENCE WAS BTWN 10-20 DEGS DEPENDING ON THE POS IN THE TURN (IN HOLDING). THE NEXT TURN IN HOLDING PRODUCED AN 'OFF' FLAG ON THE CAPT'S HSI. CONTACT WAS IN PROCESS DUE TO THE EXTENDING NATURE OF THE DELAYS IN NEW YORK (CURRENTLY LOOKING AT ANOTHER 1 HR 30 MINS IN HOLDING WHICH WOULD REQUIRE A LNDG FOR FUEL -- REASON FOR THE CALL TO DISPATCH). AS THE LIGHTS CONTINUED TO ILLUMINATE; WE WERE NOT ABLE TO DETERMINE THE SOURCE OF THE FAILURE/FAILURES AND THE PROX OF RDU (DIRECTLY BELOW THE HOLDING STACK WITH CLR WX AND AN AVAILABLE SPARE ACFT) AND THE UNCERTAINTY OF THE HORIZON AND COMPASS THE DECISION WAS MADE TO LAND IN RDU AND CHANGE PLANES. THE AUTOPLT ALSO WAS ALL BUT UNUSABLE DURING THE TURNS AND DISCONNECTED TWICE. THE DEP FROM THE HOLDING STACK AND THE ARR AT RDU WAS FLOWN BY HAND (ACFT FELT AS IF YOU WERE FLYING ON THE HEAD OF A NEEDLE) WITH AN UNEVENTFUL LNDG AT RDU. ACFT WAS CHANGED UPON WHICH WE PROCEEDED TO LGA. NO FURTHER PROBS WERE ENCOUNTERED OTHER THAN AFTER A 4 HR DELAY IN ARR WE TOOK AN ADDITIONAL 35 MIN DELAY GETTING TO THE GATE (THE PAX WERE FINE UNTIL WE COULD NOT GET A GATE UPON LNDG).