Take the cluster out.. Give it a clean.. On the connectors C1 to C3
Then put it back in.
If it still continues to do it..
You will need to undo the whole Cluster.. Very easy to do, Undo all the screws and bolts on the back of the Cluster ..
Once done, remove the Front Plastic cover.. To do this, there will be 3 more screws on the Bottom of the cluster (Where you Lamps would light up EG- Engine Light. Park Light etc)
Once off, then simply unclip the Plastic Cover.. Once off, You will notice 2 black plastic Strips.. Pull them Upwards..Once there out, you will be able to take the Gauges out.
Ok, so when you've done this, have a squizz on the Tach (chipset) for any loose and possibly burnt out wires. - If you can't find anything wrong.. Then, you might wana check the Wreckers for a new Cluster ..And take its Tacho.
If you like, I can pull apart a cluster..And take some pics so you know how to Undo them and change the Gauges
