Check the factory oil pressure switch plug by the oil filter. I had this happen before when the plug got hot and 2 of the connectors were touching. There are 3 wires on the plug. I don't remember the specifics, but seem to remember that when the wires touch the pump will either get continuous power or no power at all depending on which wires are touching. My plug got hot because I had installed an aftermarket oil pressure gauge, removed the stock sender, and just let the plug hang out of the way. It got against the block long enough and got hot.