Did you use a gasket between the oil pump and engine block mounting surface? Depending on the mating surfaces of both, sometimes you need a gasket and sometimes not. I had a similar problem last year on my I6 engine where pressure was good when engine was cold but dropped significantly once it warmed up. Dropped the pan and found I forgot to put in a gasket for the new oil pump but the original never had one that I recall. Anyway, once the gasket was in place, oil pressure stays the same whether its a cold or hot engine and I have about 20psi at idle and up to 70psi on the high.
A good feature is to add an additional Oil Pressure Gauge right at the engine block so you know what kind of pressure you have at a glance. I bought a small glycerin filled gauge from McMaster-Carr.com. The one in the photo is not liquid filled and was shortly changed out with one that is.