as title really, i have a hyperactive rev counter.

hold the revs constant and depending on how many revs i'm holding the counter waves around. e.g. if i'm holding 2000rpm (roughly) it'll bounce between 1800 - 2200rpm.

i've got it wired through the coil as it shows roughly in the instructions but i'm sure it shouldn't be this mental?!

thanks once again ppl.

Is it picking up pulses from the electric fan(s).

nope, disconnected the fan and it didn't make any difference

Probably picking up various electrical interference. Had to fit a diode between the coil negative and the rev counter on mine because it worked like a combined rev counter and throttle pot gauge. Use a 1n4001 diode wired in series in the line. Couple of quid from Maplins.
p.s. don't ask why it works. haven't a clue.