It's possible that the electronics in the tacho have gone wonky. What you basically have is a frquency to vlotage convertor that monitors the signal from the PGM and then converts it to a voltage to move the dial on the tacho. The conversion is controlled by a set of resistors and capacitors and if any of those are past their sell by date then it's possible that you could get erratic readings.

I'd go for a tacho swap to see if the problem goes away.

Form tests I've done, the MC21 tacho is about 2~400 rpm optimistic, whereas the MC28 is about 500 rpm over the top.. If you wanted to be really anal, there's a potentiometer on the tacho control PCB that you could probably use to calibrate to the correct numbers using an accurate digital gauge.

Another bike that seemed very rev happy on the tacho was the '00 R6. 16,000+ on the clock, but only ever saw a max of 15,200rpm on the datalogger and that was on the down change.