The gauge is electric, so maybe there is a ground problem. the three gauges, clock or tach w/clock, spedo and the tri gauge on the left all overlap each other. this where each one grounds together. maybe there is some corrosion between them.
Pull the instrument cluster and lightly clean the contact areas.
there is also the ground connection I believe behind the inst. cluster where a bunch of brown wires come together.
remove the oil line with a 10mm and a 13mm.
do not start the engine with this line not connected.
you will be surprised with black oil gushing on you and everything.
Charlie