Computer sluggish after new GPU

[killaconor]

MoBo= M5A78L-M/USB3
CPU= AMD Phenom(tm) II X4 820 Processor 2.80GHz
RAM= crucial ballistix 16g 4x=4g sticks
PSU= Corsair 750w
GPU= R9 290x MSI
HDD= Western Digital 1Tb
OS= Windows 8.1 x64

Hello, my brother recently purchased an R9 290X by MSI and installed it into his computer. It caused severe lag and intermittent restarting. When he installs his old Radeon HD 6850 it runs fine. At first we thought it was the drivers. So we updated to the newest "CCC 13.12" and the restarts and lag continued. Updated to the newest beta drivers 14.2. The restarts and lag continued. So we thought it was his PSU. We replaced it and the restarts and lag continued. We then would swap out his CPU with mine and start it. The restarts and lag continued. Swapped out the RAM with mine. The restarts and lag continued.Swapped out his GPU for my Radeon HD 7970. the restarts and lag stopped. We thought it was the MoBo since it was old and the drivers only went to 2010 for Windows 7. We replaced it and the restarts and lag continued. We wiped his HDD and made a fresh install of Windows 8. The lag and restarts ended. We updated Windows to 8.1 but whenever we were downloading and installing the updates it would take about an hour to do each set of 15 updates (total of about 89). We would have to "Update and restart" because we couldn't download the updates from the computers settings. After Windows 8.1 was installed the lag came back and we noticed that the "Disc" activity on the task manager would sometimes go to "90%" when idleing. At this point i think it's the HDD, but i want to make sure.

Any idvice?

 

[pyr0_m4n]

Sounds like you replaced every single part except the HDD. That probably is the issue. The more I think about it, if it happens when you swapped the GPU for the 7970, then it could be that the hard drive is failing. Most manufacturers have utilities that you can download to test the drive. I've got a good one, but I don't know how willing you are to download from random links people post.

 

[killaconor]

Before we send off the HDD we thought about swapping out the HDD with another one (Same Model) but it would still crash ( both hard and soft) so we put his old one back in and noticed that a crash (Whether "hard" or "soft") would occur every 10 minutes if YouTube had been watched on Firefox. So at this point i don't know what could be wrong. The temps run fine. Could it be a drivers issue?

 

[killaconor]

I downloaded "WhoCrashed" and ran it then this crash happened. http://imgur.com/uvhwmPO Any idea?