1999 Holden Jackaroo no oil pressure

We have a turbo diesel jackeroo 3lt 4jx1 engine. Vehicle had no oil pressure at idle but gained pressure when revs brought up. We are the 3rd workshop to recieve this vehicle. The first w/shop resealed the pick up tube seals. The second replaced these seals, both times this fixed the problem for about a week.I had spoken to a bloke from holden and he said that these vehicles had problems with oil pumps and pressure relief valves. He advised to replace both of these parts. After removal no obvious fault was detected, again holden said to replace them. The vehicle had no problems for about 200kms but now is back to square one with no oil pressure. Can anyone advise on this problem or had this problem before. Regards sean.