Hi evil85gt,
Normally the earthing for gauges are under hood, mostly near headlght area but seems you have checked them all, the last place you need to check is the connectors and the meter connection.
Some earthing wires are bunched together and plugged into a socket and then grounded with one main wire. Check those.
At the meter connectore, look for loose plug in clips, the yare plugged in but is not firm.
One more thing to check is the meter itself. The fuel and temp gauge normally uses the same earthing point so if you see any screws on the ground circuit, unloosen and retighten them again, might help.
Good luck.