As azpaul remarked most problems result in increasing transition resistance due to corrosion. Negative ground conversion has no effect to temperature gauge. There are many electrical connections on the way:
Ground to sending unit
*Sending unit to plug
Plug to cable (red/white)
Cable (rear wire harness)to plug
*Plug to main rail connector in engine compartment left
*Main rail connector to plug
Plug to cable (red/white, main wire harness)
Cable to tab
*Tab to gauge
*Gauge to tab
Tab to cable (black, instrument panel)
Positions marked with * you can disconnect and clean with a glass fibre brush, may be that helps.
Peter