I might look at the coil if I was you as the rev counter works of a pulse from the coil.
as you said it happens just after it starts jumping hope this helps
This, but also check your earths, especially as you've said there's a leak around the screen. Take 'em off inside and clean up, then check them in the engine bay and possibly replace the strappy one that goes to the coil and double up the earth to the battery to be sure.
