Floor furnace lights up and heats, then turns off and won't turn on again until the middle of the night. Replaced thermostat from a 24 volt to a 750 millivolt and same results above. Then I took the two wires from the thermostat and connected them together. Floor furnace turned on fully. After about 7 minutes, furnace turned off. Any ideas?

I believe you are having a overheat situation OR the safety limit control is defective or out of range.