The HDD is Western Digital 1TB Caviar Black, SATA 3, 7200rpm, 64MB. Bought in June 2013 like all the rest. I haven't changed any pieces in the machine since initial build up.You said western digital, but what size and model, how old is it? You need to run a diagnostic on it with Data Lifeguard Diagnostics for Windows: http://support.wdc.com/downloads.aspx
Install and run the quick test then post the results.
I'll run memtest as soon as I can. Windows Memory Diagnostics tool brought up no problems. I'll run Data Lifeguard tonight.