Sounds like an oil pressure relief valve misbehaving to me.

Quote Originally Posted by peterj1974 View Post
So still not fixed the issue but have a bit more info to run past the forum and see what you think.
I've fixed the leaky oil sender with some permatex thread sealant. I changed the oil and filter, adding 5 quarts which is what my sump takes.
Started engine - getting about 4 bar (60psi). I tested with a kit and get the same so I know sender and gauge are good.
Took car out for a drive - seemed ok initially but then after a couple of miles noted oil pressure was down to 1 bar (15psi). It would occasionally drop lower, and then come back up to 1 bar - but most of the time when driving it was showing 1 bar. But when I stopped moving (engine still running), oil pressure increases to about 3 bar. Start moving again and it drops back to 1 bar.
I thought I might need more oil in so added in another quart but still behaves the same.
Any ideas what's causing this?
Next step in my mind is to take the sump off and check things like oil pump and pick up.