AN MD80 IN CRUISE AT FL290 DIVERTED DUE TO ERRONEOUS INDICATIONS ON CAPT AND FO PRIMARY FLT DISPLAYS. NO FAIL FLAGS OR LIGHTS.

Narrative

IN CRUISE WE NOTICED THAT THE AUTOPLT WAS NOT TRACKING THE GPS COURSE ALTHOUGH THE FMS INDICATED NAV TRK AND THE AUTOPLT WAS TURNING IN A SHALLOW BANK R TURN. WE WERE CLRED DIRECT ZZZ VORTAC. ATC THEN ASKED IF WE HAD BEEN CLRED DIRECT ZZZ VORTAC TO WHICH WE REPLIED YES; AND SHE THEN CLRED US DIRECT ZZZ1 VORTAC. AGAIN THE AUTOPLT WAS NOT TRACKING THE COURSE. I SELECTED THE FREQUENCY FOR ZZZ1 VORTAC AND DOUBLE CHECKED IT AGAINST THE GFMS INFO. THE AUTOPLT WAS ALSO NOT MAINTAINING ALTITUDE THOUGH INDICATING ALT HLD. THEN WE NOTICED THAT MY (CAPT) PFD WAS INDICATING LEVEL WHILE THE FO'S FLT DIRECTOR INDICATED A L BANK TO RETURN TO COURSE; WHICH WAS CORRECT. AGAIN THE AUTOPLT WAS NOT TRACKING THE COURSE TO ZZZ1 VORTAC THOUGH NAV TRK WAS INDICATED. IT THEN APPEARED AS IF THE CAPT PFD WAS INDICATING A L BANK THOUGH WE WERE LEVEL WITH THE HORIZON AND THE STBY ATTITUDE INDICATOR. THE FO'S PDF NOW SEEMED TO MATCH THE CAPT PFD; BOTH ERRONEOUS INDICATIONS FROM TRUE LEVEL FLT. AT THIS TIME WE HAD NO FAIL FLAGS OR LIGHTS. CONSULTING THE QRH; SWITCHING TO THE AUX GYRO DID NOT REMEDY THE SIT. NO QRH PROC RESOLVED THE DUAL ERRONEOUS PFD/LOSS OF VG INFO SIT. THE FO CALLED DISPATCH WHO PATCHED US THROUGH TO MAINT CTR. THEIR LAST SUGGESTION WAS TO TURN ON EMER PWR AND THEN TURN IT OFF AGAIN. THIS PROC RESOLVED THE DUAL ERRONEOUS PFD SIT. WE RETURNED TO ZZZ TO LAND IN A VFR SIT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THE PRIMARY FLT DISPLAYS WERE SINGLY AND AT TIMES BOTH ERRONEOUS IN THE PITCH ATTITUDE. AT NO TIME WERE THERE ANY FAIL FLAGS OR LIGHTS INDICATING A FAILURE. MAINT RPTED THE CAPT'S #1; FO'S #2 AND THE AUXILIARY VERTICAL GYROS WERE EXPERIENCING INTERMITTENT MOMENTARY MALFUNCTIONS. THESE WERE MECHANICAL GYROS AND NOT LASER REFERENCE UNITS. MAINT DID NOT ADVISE WHY THE AUTOPLT DID NOT TRACK PROPERLY; BUT MAY ALSO BE CONNECTED IN SOME WAY TO THE GYRO PROBLEMS.