Sounds like a wiring fault to me to be honest.
How is your loom wired up, just the standard setup? In a normal cossie there are 3 multiplugs for the loom in the engine bay and these cause all manner of issues from time to time which leads to most people cutting out the connectors and soldering the wires together. I suggest that you either clean up the connectors or next time you have a rev counter issue that you stop try and move the wires and plug about and see if that cures it.
I had exactly the same problem on my car for a long time, and it wasn't until i got an secs monitor that it became apparent why i had a misfire, the SECS monitor is digital and wired into the ECU, this showed double the RPM of the rev counter, and why it was misfiring. This is the first thing I'd check as it won't cost you bugger all, hope this helps.