I had a 89 ranger and have known others who have had the same problem. I chased it as an electrical problem because it was eratic like that. What I found was that in the late 80's and early 90's Ford made the oil pan gasket to over hang in the oil pan and it deteriorates over time stopping up the pick up screen. That is why when you stop the engine or in some circumstances it will drop to a low pressure and then come back up. You may try a sending unit but I am betting that you need to drop the oil pan and replace the pump, pick up screen and gasket. Two people that I know had their engine lock up because they thought it was electrcal and would get good pressure readings so I would not delay a fix. Its not hard or expensive to do Im betting money on the screen. Just my 2 cents.