So, the reason was that one of the X9SCM-F NICs couldn't work with ESXi 5.0.
That's why you prepared a custom image with the proper custom drivers and installed it on both X9SCM-F and good old X7SBi, right?
And, fortunately, the same custom drivers worked fine with ESXi 5.1 for both MBs, right?
I would say that VMware should pay a little attention to the situation with e1000 (one of the best NICs, so far). 