Tach works part of the time

How are your other gauges working?  Have you measured the voltage coming from your battery?

At a guess, I'd say you have a bad ground or other connection in the instrument cluster.  It takes 30 minutes of heating/vibration to finally make contact like it should.  With electronic gauges like the tach, the voltage difference between working and not working can be pretty minute.  I'd suggest you go through the wiring from the sender to the tach, section by section, cleaning and tightening everything and, when you get to the instrument cluster, look for corroded or “crystallized” looking solder connections.  Any that look suspect can be re-heated with a soldering iron or gun to “freshen them up.”