Outer Limits 4x4 Board

noticed latly that when the revs get up over about 3,000 that the oil pressure drops off dramaticly from like 3/4 to under 1/4 if i hold the revs up for more than about 30 seconds or so... changed the oil/filter and it did nothing, has the right amount and type of oil in it... wondering what else could be the cause???

seems that something is sorta blocked and is restricting flow at higher rpm, anyone had anythin like this? and what ended up bein the cause??

cheers