[Tom Goldman]

Usually the cooler pressure issue is on cars using too small a cooler or undersize lines to the cooler.
Some transbrake valvebodies also up the line pressure fairly hi and aggrivate the problem.

I've only ever had to restrict 1 th400 that had this problem.
Heres a picture from a forum I found a few years ago.
Restricter size preference seems to be between .090 and .150

I would check the cooler pressure and the cooler for restriction before making any mods to the pump.