Low oil pressure

I have a 1991 aerostar van, it has about 119,000 miles on it it set in my brother in laws yard for five years, he parked it because he got a new work van,
it has a 3.0 motor I have fixed alot of things on the van but my oil pressure drops after the van warms up but if I cut the engine off wait a few minutes and start it back up the pressure comes back up for a few seconds, I know it isn't the sending unit because I can hear the lifters begin to tap, is it the oil pump or something more serious.