Temp gauge/tach shutdown

Since you are having problems with two gauges at the same time and one of them seems to not be accurate, my first guess would be an electrical problem.

Check the connections at the temp sensor and at the fuse block for the instrumentation circuit.  Sounds a lot like a loose connection or a short in a wire somewhere. After that it gets a little more complicated without seeing a schematic.

I once switched the wires between the oil pressure sensor and the oil pressure switch, boy did that make things go wonky until I figured it out.

the Chinese pin connectors have no gel or o rings so they can get corroded from dirt and moisture. and can be a issue and remember check the grounds check to see if these two gauges run in the same harness . good luck