Your plight reminds me of this individual who just about changed everything trying to cool things down. Granted, it is a different animal but, actually the application is even more severe than most cars regardless of power output. In his case after tearing apart things replacing stuff he finally came round to the idea his pump was the culprit.
For your situation it may indeed be something else but if you have the time to read it all it is a good study or relatable:
Runs Hot - Out of Ideas
