250 Temp gauge fluctuates

[playinsafe44]

I'm working on a '73 chevy with a 250 straight 6 in it. The heater hose blew, and the head cracked too.... Well bought a reconditioned head and put it all back together, but now the temp gauge is fluctuating between 190 and 230. It had a sending unit in the thermostat housing, and one in the head. The one in the thermostat housing got destroyed, and now it has an aftermarket gauge in the head. Any thoughts on what is going on? It runs and drives perfect.