Electrical or head gasket

The car was overheating and warped the head. Replaced with new (used) engine. Ran great but would eventually start missing until it would not turn over at all. I replaced the spark plugs and it started right back up like new. Until it started to overheat once again. My mechanic said head gasket again and put stop leak in my brand new radiator. I had it flushed and it ran good until the plugs fried again. It repeats that same cycle. Starts to overheat after about a month of running like a bear. Then starts to lose a cylinder at a time starts to miss. Then will not start at all until I replace the plugs? They keep telling me head gasket but one mechanic said temperature sensor was gone and he unhooked it and ran the wire direct to my radiator fan to stay on all the time. I think that is what is causing most if not all of the havoc. Any help is appreciated. Nothing in the oil, no milky antifreeze.