You might want to check for a loose wire at the sender unit. As I understand it Ford for years has used an ON/OFF type pressure sending unit in conjunction with a guage. The guage is in place of an idiot light. Just like the light you should either have pressure or not. You should notice the guage never moves from idle to high RPM and never fluctuaits with oil temperture change. The sender may be on its way south. I guess it is possible to disslodge some sludge with the oil additive. If you remove the sender unit you should find the build-up there if thats the cause. Let us know what you find. Chris