MGB Cooling System

I have a 1979 MGB with 35000 miles. It has an electric fan. The fan wasn't coming on but I could jump the wires on the fan switch and make it run. I replaced the fan switch. It worked for about 4 days and the fan stopped coming on again. I can still jump the fan switch and make it work but the fan switch doesn't seem to be working. I changed the thermostat and the overflow cap has been changed (there is no radiator cap so the overflow is the sealed cap). The hoses do not collapse. I can find no leak. I do not know how to test the relay but I'm not sure a relay could be the problem since I can jump the fan switch. Is it likely that when I replaced the switch I did so with a defective one or is there something in my system causing the fan switch to go bad and, if so, how do I locate it? Or do I have a relay problem?