Quoting toybreaker:
The symptoms you are describing with your oil pressure fluctuating can be caused by a pressure relief piston hanging in the bore.
Do you know the history of the oil filter adapter?
Has it been "modded" around the oil pressure relief port?
Before tearing things down, it'd be worth pulling the filter adapter and checking the pressure relief valve piston and it's bore for a burr/debris and/or a broken/weak spring or some such.
I've seen a modded one create this exact same scenerio, and the poor guy had it hang on a brandy new build, and it cost him all the rod bearings as well as the crank. (again! /ubbthreads/images/graemlins/banghead.gif )
Great info Toybreaker. What do you suggest doing to the relief valve piston? Just sanding it down a little bit if there is a burr/debris?