1995 Dodge Caravan heat

If the heat comes and goes without touching the temperature control, sometimes it gets air into the cooling system, its usually from a small leak, a pressure tester or a chemical is used for picking up traces of combustion gases in the coolant, this is to test each head gasket for leaks, did you ever have the cylinder heads checked? That may be the problem, as air enters this closed system the first sign is intermittant heat. You should makie sure there is enough coolant in the entire system, a radiator cap leaking can also cause this.