There is ONE set screw that holds the oil pump in place, near the top of the pump shaft. The set screw has a lock nut on it. Loosen the lock nut first, then you can loosen the set screw and remove the pump. I have a '59 235 and I had the same problem you have. Oil pressure came and went. After checking and eliminating all of the easy possibilities, I finally replaced the oil pump. Now it's good as new, with steady, high pressure. The old pump showed no evidence of wear and tear, so the fix is a mystery to me.
