The central three gauges on my dash are all slightly random. The battery meter fluctuates between the bottom and top of the dial (I've checked the battery with a meter - it's fine), the fuel gauge shows a very, very rough idea of what's in the tank, but it'll do it in about 3 stages. Full, half full, empty. Not ideal, but I just keep filling her when it's at half full!
The temp. gauge is similarly random, when it works. Sometimes it'll jump from cold to hot and back again.

Any ideas? I'm thinking there may just be a common fault somewhere as all these gauges are housed so closely. The speedo/odometer and the revs/oil pressure are both working fine.

 

Do the gauges fluxuate when you press on the cluster as a whole. ( is your dash loose) the gages recieve electricity thru a metal and plastic over lay type deal that each gage ties into with screws making contact with the overlay so if you can press on it and see variation make sure the cluster is properly seated.