Faulty Rpm Indicator

[Jose L. Ichaso]

35 HOURS FLYING AFTER INSTALLATION OF BOTH OVERHAULED ENGINES, THE RPM NEEDLES STARTED TO MOVE UP AND DOWN A LITTLE WHILE ON CRUISE. AFTER THAT, WHEN I FIRE UP ONE ENGINE, IT SEEMS TO INDICATE THE CORRECT RPM NUMBER, BUT AFTER I START THE OTHER ENGINE THE NEEDLES START TO COME UP AND DOWN WILDLY ( IT HAPPENS WITH EITHER ENGINE ).THE FREQUENCY OF THE OSCILATION OF THE NEEDLES DEPENDS IF THE ENGINES ARE AT THE SAME OR CLOSE RPM WITH EACH OTHER; LOWER THE FRECUENCY AS CLOSER THE RPM ARE BETWEEN THEM.
THE INSTRUMENT SHOP HAS TRIED OUT EVERYTHING, THEY SAID THEIR PARTS SUPPLIER NO LONGER CAN SHIP THE PARTS NEEDED, AND I HAVE TO SEND THE INDICATOR TO U.S. ( I WILL TRY OTHER SHOP)
ANY SUGGESTION OR APPROACH TO SOLVE THIS? HOW MUCH WOULD BE THE OVERHAUL COST, AND RETURN TIME? WHAT ABOUT DIGITAL INDICATORS INSTEAD?

THANKS FOR ANY HELP,

JOSE L. ICHASO
1969 337D,NORMAL ASPIRATED.