BW 35 Automatic Gearbox problem

[PGAHOWELLS@domain.elided]

Hi all
I think I have finally cracked it.
I think I lose the pressure at speed because the box overheats and the fluid
becomes too thin and by passes some seals somewhere, possibly governer which
loses pressure hence involuntary high speed downshift.
(When I leave the car after a run it has a hot oil smell. Also my oil cooler
pipes get very hot, that is compared to my old Discovery. Additionally I have
a small leak from hot oil coming up the filler pipe).

So at least I find out what happens when an Autobox overheats. I have spent
ages trying to get the answer to that one.

Why is it overheating?
Oil cooler flows good. Pipes have been renewed.

I think one of the pressure relief (check) valves is permanently open, meaning
fluid is going straight into the sump instead of being pumped around the
cooler.

Is it the converter relief valve or the other one? 

So in a few days time, of with the sump pan, and out with the valve block to
check.
Has anyone done a valve block? Is their anything in particular to watch out
for. Do I need a plastic bag round it to catch the ball or disc and spring
when the separator plate comes off. 
Would just prodding  the check valves through the hole or twisting them gently
with a thin screwdriver solve it.
Could the valve be missing or seated on a soggy spring that blows out the
pressure too soon.
All verbal donations gratefully received before I do it.
Peter H