I first noticed the rev counter playing up when sat at a junction and put indicator on, the rev counter dropped then bounced around a bit. Now, sometimes I start the car or idling the rev counter doesn't work at all, then it'll start working, sometimes needs a tap on the dash, but as you apply the accelerator it starts to climb then drops down and back up, bounces about a bit but starts working (or seems to be) above 3-4k! What's going on?