I have heard of these GM 360/390 R boxes occasionally loosing pressure as the fluid get's warm. I remember coming across it whilst researching the fluid changes. As a precaution I changed my main pressure regulator (the common denominator in terms of fixing) on the valve body whilst I had the pan and filter off. you also have to loosen (emphasis) on loosen! SOME of the retaining bolts of the valve body down from the underside of the transmission, so the regulator clears the tranny casing. I remember there were two bolts that couldn't be touched (but had the same bolt head design) if you inadvertently undo those, you could have a valve body separate and end up chasing ball bearings and check valves around the floor. So my post is to say that the regulators can fail, it is often associated with temperature build but to be careful installing a replacement regulator.
I used to have a printed copy of the rebuild manual for these, I'll try dig it out and scan the relevant page.
There are resistance checks that can be done on the regulators at different temperatures with a tolerance of +/-15%, the regulator cost me around €60 if I recall correctly. At work now, but will search for the manual when I get a chance.