Erratic readings on your fuel gauge or water temperature gauge could be down to a faulty instrument voltage stabiliser.
Rather than replace it with another using the same old-fashioned technology, you could fit a more accurate and reliable solid-state version:
http://bob_skelly.home.comcast.net/~bob_skelly/voltageStabilizer/voltageStabilizer.html
There are ready-made replacements available on ebay, like this one (specify negative earth and terminals required):
http://www.ebay.co.uk/itm/SOLID-STATE-V ... 1e79f661ee
Alternatively, you can make your own very cheaply using the following parts, plus some wiring and lucar connectors. The case can be a new ABS case or just open up and re-use the old one.
Solid-state voltage regulator:
http://cpc.farnell.com/stmicroelectroni ... dp/SC08361
Capacitor (needed only on input side):
http://cpc.farnell.com/multicomp/cb1v33 ... dp/CA07930
Box:
http://uk.rs-online.com/web/p/potting-boxes/2250061/
Unit markings:
B is battery, input from ignition.
I is instruments, output to gauges.
E is earth.