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I too have had similar issues (steep hills or soft sand). It is if the transmission won't drop down and I have to manually drop it down to 1 to get the revs up. I find taking the overdrive off...

on my Patrol going up hill with heavy load would cause the semi limp mode , no error code or check engine light would show up.. eventually going to a bigger exhaust fixed it.

How was the engine temperature?

The engine slows down when getting hot, may be good to have a check on the radiator...

Try swapping the Air con relay with the ECM relay,
maybe re do a few earth straps as well
Might want to fit a catch can as well