I take it the engine sounds ok no rattling or abnormal noises and it runs ok
before ripping it apart I would change your oil filter, it has been known that if you use a non oem filter you could have problems.
I would have so good look at the pressure sender and the wiring as well may be try get a second hand press sender from ebay or a mates one :-)
Failing that your looking at pump and or pressure relief valve as your next option if you are going in to the engine case I would change pump and refresh the relief valve assy.
Has any work been carried out on the engine lately
Hope this helps