That drop from 55-60 to 30+ psi going from neutral to drive seems excessive. When you pull the present pump, I'd check all the internal clearances.
BTW, oil temps can be a good bit different from the coolant. Is petroleum oil being used? A full synthetic will typically be more stable over temps so that might help; a heavier synthetic at both weight numbers will stay thinner down at cold start temps than a petroleum oil, so that is another option for you.