05 800 rev not cooling

There are a couple of possibilities... If an old thermostat had come apart, dropping the lower piece into the cooling system, it could cause a restriction. Sometimes that little piece makes it all the way to the water pump, gets caught between the impeller and the housing, and grinds away at the housing enough to cause excess clearance and not allowing the pump to work properly.
Another possibility could be a bad cylinder head o-ring, causing air to be forced into the cooling system and pushing the coolant out and stopping circulation as a result of air bubble at the pump.