MG Car Repair: A+ Series Oil Prfessure, oil pressure gauge, pressure relief valve

Question
I have been dealing with too much oil pressure since I installed an A+ series engine into my 1965 Sprite.  At start up idling I have 85 psi then after warm up the idling pressure is closer to 60 psi. Driving at 55 mph the oil pressure holds above 75 psi and at times it is barried at 100 psi, it will vacillate between 60 to above 80 psi most of the time.    I’ve tried various things to fix this problem because I been told that the oil pressure should be no more than 60psi at 3000 rpm.  I’ve verified that the oil pressure gauge is working properly. Then I’ve remove the plunger in the oil gallery and replaced it with with a 9/16 ball bearing and smaller spring which only increased the oil pressure.  So I’ve returned to the plunger and cut down the oil pressure spring now I’m nearly ¾ of an inch shorter than the stock length spring and I hold about 60 psi but the oil pressure will again vacillate from 20 to 60 psi even when I’m driving above 60 mph.  At idle I keep about 20 psi which I’ve read is ok for idling.  This Oil pressure concerns me and I would like to get somewhere normal readings.  I bought this “long block” A+ Series engine from a popular and honorable English parts company and I think it is a late model engine from an Ital, Morris Marina, or Austin Metro.  
Can you provide me with ideas as to why I’m having this problem?  Could it be that a high pressure oil pump was added?

Answer
This is a most unusual question.  Normally the problem is insufficient pressure but you have too much.  20psi is fine at idle but 75psi on the motorway is excessive and could indicate a restricted flow rate on the pressurised side of the pump.   

I am particularly concerned that the pressure oscillates between 20psi and 60psi.  This sounds like a sticking oil pressure relief valve.    

Other possible causes include failure to bleed the air from the pipe to the gauge or a blocked oilway.

The oscillations could be caused by a fractured oil pick-up pipe that sucks air when the oil level in the sump drops, a collapsed oil filter element, or a spin-on oil filter with an internal one-way valve that is not functioning correctly.  However, none of these would explain the excessively high pressures you are seeing.

I have a feeling that the only way to find the problem is by stripping the engine and checking every oilway.  Tedious, but there is obviously something wrong, and if any bearing is receiving insufficient oil you could quickly run a bearing or the engine could seize solid.