Checking the specifications for the processors (http://ark.intel.com/compare/53480,65520) the PCI Express* revision of the Pentium processor is 2.0 and for the i5 is 3.0. Since your motherboard is revision 2.0 using the Pentium processor your video card is downclocked and it works at 2.0 with no issues. Then, having installed the i5 processor as we know it works at 3.0 and the video card until that point can work at 3.0 also. The problem is that the PCI Express* bus of the motherboard doesn’t support 3.0. If you have the possibility, test another motherboard that be PCI Express* 3.0 compliant with the processor and video card and see if the issue persists.