1993 Dodge Spirit Over Heating

I recently purchased a 1993 Dodge Spirit in Feb. When I purchased this vehicle it had had "stop leak" put in the radiator whhich caused the heater core to go out. So I replaced the heater core and had the radiator fixed also. Now since a month ago I found a hairline crack in the water pump housing. So I patched it with some "JB Weld". Then it was working fine heat gauge didn't even get close to the middle, but now as of acouple days ago my vehicle has started to over heat rapidly and I have to keep putting water in the radiator. But I have found out yesterday that if I keep the defrost on high heat then the radiator holds water and does not over heat. I thought it was the water pump but my uncle who is a handicapped retired mechanic told me that it might not just be the water pump. Also the cooling fan comes on even the it is wired straight into the fuse box's.