You've clearly replaced every single component in that system and, at this point, it should be operating perfectly.........with one glaring exception.

When you change the airflow characteristics via the use of different fans/shrouding........etc, the result must be considered an experiment. You claim that "plenty of hot air is being emitted by the fans", however the engine might beg to differ with you.

Airflow across the radiator is a complicated subject at higher speeds and the fans may impede this airflow rather than enhance it.

I'd change back to the stock setup and see if your issue disappears.