Rev counter bouncing

[A900ss]

Went out for a blast today and on the way home the rev counter started bouncing +/- 500 rpm from the correct rpm.

The car was running fine.

Funny enough the 'bouncing' was worse over bumps. When the car bumped, so did the indicated rpm.

I've searched but can't find any definitive answers. I'm thinking about replacing the coil (looks original from 1997) but any other input welcome.

Like I said, car runs fine, it's just the bouncing rev counter that's the problem.

[A900ss]

Went out today and the rev counter was good as gold, same as it was before yesterday.

I can only assume that I got some water somewhere it shouldn't have been as I washed the car yesterday.