What brand of oil filter are you running? There's something causing pressure to vary considerably at some point, and while the oil filter is very unlikely to be any part of it, it's easy to change; they have a valve in them to bypass the filter if they get full and it's just a thought.

More likely is the pressure regulator thingy, the part that has the rubber shoe on it and keeps tension on the timing chain. This particular part(s) can likely be replaced by removing the timing chain only (can't remember) without removing the balance shaft chain. It sounds like it may be that, but I'll let others have a look at what I'm thinking and comment further.

John B.