My tried and true D5B is getting pretty slow to get to move after cold start or hot re-start. After it engages everything is normal but it can take up to a minute to get it to move. It has always been slow to get to move but it seems to be getting worse. I replaced the trans pump years ago but it didn't make any difference. I have noticed that checking the oil while running on a cold start that oil seems to be full of bubbles but not foaming. The oil is fresh and clean the filter shows no sign of contaminants. Torque converter maybe?
Have you looked at the obvious things like an air leak in the pump suction side.?
I have given everything a look over and didn't see anything obvious. I'm going to bring it back in , pull the floor and tighten anything that can be tightened. I hope it as simple as that. This old girl doesn't have any leaks so checking for pressure leaks is easy. A suction leak isn't as easy.
It may be time to go beyond inspection and replace the suction hose, gaskets, O-Rings, hose clamps, etc. You never know what you may find. Is there a suction screen.? Is it clean.?
Yes there's a suction screen and luckily it's accessible under the floorboard unlike the D4H that I just got finished with that is inside the trans housing. The mag screen was clean and a filter cut and press showed nothing. I think that your right. I'll go through and replace orings and clamps and see what that turns up.
Check all your connections from the pump to the trans replace the rubber suction hoses replace all the O rings in your strainer housing if the don't work reseal or replace trans pump again years of sucking air will take a toll on the pump