Ive got that problem atm :S
Only thing I can think of is possibly a loose or faulty wire somewhere.... maybe? Ive pulled out the cluster and tacho and completely pulled it apart looking for a bad solder or loose wiring but found nothing, have given the dash a hit which fixed it momentarily. lol
But it has slowly started happening more often for me :S
Other note... I have PFC with controller and it always shows the correct engine revs, even when the taco is going crazy. So for me its either the cluster itself or a wire between the ECU and cluster. I was going to buy a new cluster but I'm sure I read of someone with the same problem and a new cluster didn't fix it. I haven't been bothered to look at wiring as yet though, Anyway - I'd say its either bad wiring or the cluster. good luck.