what make of oil filter are you using?,only use a good quality filter,such as unipart or britpart,because some filters have a weak spring inside the filter,called a bypass spring,its job is to bypass the oil rather than filter it,if the pressure was to go to high,if the spring is weak,poor quality,then the oil is bypassed,not filtering it properly,then looses pressure,just a thought mate,as ive seen this once or twice.
