Do you know if the tach is electrically or mechanically driven? I had a similar issue on a different brand of tractor. After running the tractor near pto speed the tach would climb to 3000 rpm but the engine speed did not change. Throttled the engine down to idle but tach would read 2000. Shut the tractor off and would be OK until I raised the rpm up. Would happen intermittantly but fortunately it eventually acted up for the tech. They had to replace the instrument panel. Thought it might be electrical in nature but it turned out my tach was mechanically driven and something inside the tach was acting up. Fortunately it was covered under warranty.