So a PCIe 2.0 x16 card.
I'm just finding it hard not to think that with a new mobo and a 750W PSU that there isn't something wrong with the 6870 itself.
There's just no reason that a 2.1 wouldn't work in a 2.0 slot.
When there's nothing wrong with the slot... or PSU... or mobo.
I think they've been sending you back the same XFX card...