Mine still done this before I took it to bit's to restore it,
I tried 2 different sets of clocks in it,
GrpA coil then a New Genuine Ford one,
New HT leads and coil lead,
New Ignition amplifier,
Checked all 3 of the mutliplugs,
New Phase sensor
New dizzy cap and arm,
and it's still the same.
I had a missfire around 4500-5000rpm and the needle would bounce right off the clock.
On idle it used to jump a little, no more than 500rpm either side....
I'm putting all new everything on the car whilst I'm rebuilding it and I'm gonna replace the wiring from the coil to the rev counter and the crank sensor, the only things I hav'nt done yet....