The tachomoter drops down to near 0 when the car is idling. It's clearly idling at it's normal idle speed, somewhere around 700 rpm. It also seems a bit inaccurate at the high end now; for example, if I'm driving 65 mph, the tach is reading about 100-200 rpm more than usual. I have a manual transmission.

I believe this started around the time I last changed air filter. I don't know whether there's any connection between these events.

So, I'm wondering what might cause this, and what the solution might be.

Could be a bad wiring, could be your dash's circuit board. Could be bad VSS. If you cruise controll works than it might be the circuit board.

Sam