Well, took the PC to the repair shop and eventually found out it was the mobo after all.
Computer was tested for 24 hours with a PC technician at a who found the following faults with the mobo. pci-e16 slot not securing graphics card correctly. Possible power supply problem from mobo to ram slot, and faulty northbridge accordng to an error code they got whilst testing it.
So, back to asus the mobo goes to be replaced for a fourth time.