The issue appears to be that the processor is not being properly detected by the motherboard or operating system.
Check how the processor is reported with the Intel(R) Processor Identification Utility http://pixel01.cps.intel.com/support/processors/tools/piu/sb/CS-014921.htm
if the Intel(R) Processor Identification Utility also reports 8 cores as the Device Manager, you may need to check with Microsoft* to see why the windows 7 system information is only detecting 2 cores.
The Intel Processor Identification Utility reports Intel Core i7 CPU 860 2.80GHz. Intel QPI 4.80 GT/s 1333 MHz NO L3 Cache and only 4 Cores.
I just checked the specifications for the processor and I can see that tje processor has only 4 actual cores, with is what the Intel(R) Processor Identification Utility will detect, the other four cores are reported by the operating system due to the Hyper-Threading Technology enabled on the processor.
At this moment I would recommend you to test a fresh operating system installation and update the BIOS of your motherboard in order to make sure that the processor is being properly detected by Windows* and the motherboard.
How many showing up in Task Manager for CPU usage not Device Manager?
Reset the CPU in the socket and check the socket for damage also fitting a cooler too tightly or incorrectly that are by nuts & bolts could have damaged the CPU and or board.
Hello,i had the same problem myself this week i fitted a i-7 870 in my Gigabyte p55-usb3.All the benchmark software said it only had 2 cores and 4 threads.One benchmark said it was a i-3 530 Havendale.I tryed all sort of things to get it to work but in the end i got it to work by doing a fresh intal of windows 7 which is what i was useing.I dont no why this happened in the frist place but the benchmark software shows it ok now and so does windows 7.Hope this is of help,good luck from dave.