I did try everything that was posted anywhere:
1 - Try 1 board at a time
2 - Increase temps inside
3 - Flash all firmwares + enablecores

And I am yet to get an answer from KNC if the problem is of pure hardware nature or it can be fixed by a new firmware. And I need to know that so I can decide if and when to RMA.

Btw I don't expect to get any resolution here. I'm just using this space to vent my frustration and dissatisfaction with KNC.
I've tried my best to be reasonable over the phone and was practically begging the guy to step in my shoes for a while and realize that sending the boards back is adding insult to injury with the 2 weeks delay I had.
I know they can organize an advanced RMA - it's not rocket science. They just don't want to for reasons, which are not genuine.