I am tired at the moment and my mind is not calculating correctly but here is a thought or two.

Try by-passing the tranny cooler in the radiator. it may have blockage or restricted flow.
Are the cooler lines kinked anywhere?
The clutches may have been dry and need to soak up some fluid befor they will grab properly.
There may be blockage in the valve body or a sticking valve.
Is the internal linkage in the tranny pan hooked up?
Last, was the tranny PROPERLY assembled?

There is enough good in the worst of us and enough bad in the best of us that it does not behoove any of us to criticize the rest of us.
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Be yourself. If you are ever lost, It will be much easier to find yourself if you know who you are!