So I would lean towards two things, the 40 W as you mentioned and checking the oil pump for any sort of grit in the relief valve. Just to be sure I was loosing some of the pressure being created by the pump in the first place.
I see an idle pressure of about 75 psi on start up and about 7-9 psi once up to high temp.I will get a red light at idle which is set at about 850 rpm.
that you are right at the edge of the acceptable numbers. Your hot psi of 7-9 is low for idle,