my CPU is a Intel i5 3570 IVY BRIDGE (ivybridge supports PCIE3.0)
now the problem is that when I put my VGA in the PCI Express 3.0 x16 slot (PCIE2: x16 mode), my monitor says: NO Signal
When i put HD7870 in the other slot ( PCI Express 2.0 x16 slot (PCIE3: x4 mode) ) everything works fine...
Why I can't use the PCI3.0 x16?
PS: NOT a monitor/cable problem, tried with HDMI and DVI cable.
It depends. Sometimes they can be straightened. I would take the board outside the case and test again. If still the same, uninstall the CPU and use a magnifying glass or take a high resolution picture for inspecting the socket.