I have a 90' Ford ranger 2.9L 5speed. When going a fairly long distance on the highway (10 or 15 miles) at 55mph, and then coming to a stop, the oil gauge drops to low and the engine acts as though its going to die. It's then just fine once you take off. My father's 88 Ford F150 does the same thing but it goes away if you rev the engine. What's wrong