Tacho Going nuts

[Simachine]

I was wondering if anybody could help me. I have a 96 5ltr statesman. I bought it a couple of months ago. When I got it it ran fine on idle, had a bit of a vibration around 2000rpm and that was it. It slowely started running rough and surging. I have replaced, dizzy cap, leads, plugs, filter, serviced the motor and trans. Was runing good for a while and is now back to surging and running rough on idle. I've noticed when it is running bad (intermitent, it dissapeared after twisting the injectors and smacking the ignition module and wiring stuff around it) the tacho bounces around when accelerating and goes nuts at idle (bounces up to 2-3000rpm) but the car is not reving.
Does anyone know what this is?

[hako]

The ignition module can cause this problem but there is no guarantee.

[baza]

when the tacho needles start going nuts it sometimes indicates a faulty ignition coil, cheaper than ignition module as well. maybe b4 rushing out and spending money, get some carby-throttlebody cleaner and clean the throttlebody . also remove the IAC motor from the t..body ,clean it and make sure the pintle [ spring around it ] shaft can move in and out . need to use a bit of pressure to move it. dirty IAC and throttlebody can cause rough idling and surge. hope it helps........