I had a similar issue, it would not lock up the ESXi host but it would only pass through the card on the first power on of the machine and not on subsequient power ups. I found an option in a white paper about RAID cards and tried it and it solved the problem for me. Add the following to your configuration file and try it, obviously with the correct number for your pass through device.
pciPassthru0.msiEnabled = false
I now add this line as a matter of course to all GPU pass throughs and most others. It will be deleted if you remove and re add the hardware so watch out for that.
On another note I have tested Crossfire on my x58 board and it works! I have 2x 5770s' working in CF but with a bridge fitted as the drivers would only enable CF with the bridge in place. I am going to borrow another 6770 as the one I have has no bridge connector so should allow software crossfire. If this works (I see no reason why it would not) then I can have 2 VM's each with 2 cores and 2gb when needed and change to 1 VM with twice the power for gaming when needed.