For the last few weeks my tach needle sometimes jumps up to just above 4000 rpms and stays there.
Sometimes it drops down after a while and acts normal all by itself - or I have to stop and turn the ignition off and then its fine.
Me too. Stuck at 4000-rpm after cranking. Seems like a universal problem but I don't want to start tinkering with it until I see what others have done to correct the problem.
Unless you have two years or more of electronic experience, all you can do is swap out the Tach. There is a circuit board with transistors, resistors and capacitors all soldered in place. You need to be able to troubleshoot the electronic components to fix it.