Okay, let's back up a step. Trouble shooting is about evaluating step by step, or a process of elimination. Deal with some basics first. Perhaps I should have emphasized the oil viscosity thing more.

What oil viscosity are you running?
Are the numbers you've shown above at cold or warm?
Have you checked the oil level?
Is it low or high, or just right?
Do you know if the proper dip stick was used at rebuild?

In a sense you can't "check" the pressure reliefs except by what you're already seeing. There's one at the oil pump and there's one in the filter (assuming you're using a good quality filter). While the symptoms are of pressure relieving (dropping), it could also be caused by oil starvation. Oil starvation could happen from too viscous of an oil, a partially plugged oil filter, contamination (blockage)of the oil pickup screen or other part of the oil circulation system, low oil level, or oil aeration of the oil (result of too full).

I'd like to ask if it's been doing this from the beginning, but you've indicated that you've not operated it much, and that you're operating differently now than before. Not much apples to apples there.