Have 96 Cummins diesel, looses oil preasure when you are driving or rev the engine if idling oil preasure seems to be normal... has anyone else had this problem???

Now that's odd but evidently the fellas that put together the Haynes Manual for the 1994 thru 2001 Dodge pickup #30041(available at most chain type autoparts stores) must've had a similiar problem because they describe in detail replacing the oil bypass valve on the oil cooler.
Of course you know to be sure the oil level isn't too high or too low in the oil pan. You might also check to be sure the oil pan doesn't have a big dent in the bottom.