x1000 onboard network opensource driver in progress

What if you change your interrupt code to always return 0?

And/or change interrupt priority (either to very high or very low)?

Edit: also I would add a static counter variable to the interrupt and every 1000 counts or so debug-output it to serial (unconditionally = even if "interrupt is not for me")). To see at lockup what happens: maybe there's interrupt-flood.

I would generally also look more at what happens during lockup (the OS is still likely doing/executing something) not just how to reproduce it.