Looks like we're on the right track now. All I can suggest for the cooler circuit is to try to do a reverse flush similar to reverse flushing a radiator. This would mean disconnecting both pipes to/from the cooler and finding a way to flow fluid through it in the reverse direction.
I've spoken to our local (and very good) specialist Keith Marshall who confirmed it is an oil pressure problem and his opinion is that the above will not improve things much. However, he did suggest that to prove it one way or the other, you could bypass the cooler for a day (loop a temporary pipe between the inlet and outlet to the transmission or around the cooler) and see if it makes a difference.
A reputable transmission specialist will likely have a rig for reverse flowing the cooling circuit on the car and so shouldn't break the bank to get done if the bypass thing makes a difference. Keith said that when coolers get clogged like this they tend to replace them with a new cooler. Luckily, aftermarket coolers are generally not hard to fit and not too expensive.
Failing the above, I'm afraid he said that it becomes a "transmission out" repair, so it's worthwhile exploring the above alternatives first.
Keith's one of the best transmission people around although it's always possible he could be wrong - lets hope so. I believe Clark Steppler (the source of info for the previous link to se-r.net) is a Nissan tech guru without peer so his opinions on this do carry a lot of weight.
If need be I could scan the transmission removal/refitting pages of my Haynes manual and email it to you. You would need to let me know the transmission type you have (listed on the build plate) though it should be a RLF03A.
Good luck and keep posting your findings
