Different Blue Screens mostly with ntoskrnl.exe

[iBaer]

Hey guys,
I am getting quite frequently some blue screens, all with different stop codes and I don't know what to do anymore.
The bluescreens happened before and after I made a clean install of windows 8.1 last week.
Memtest told me after 8~ hours that both of my RAM blocks (2 and 4 GB) are fine.

I received the following BSODs since the last clean install (info from BlueScreenView):

Spoiler

070514-47750-01.dmp 05.07.2014 22:10:05 DRIVER_IRQL_NOT_LESS_OR_EQUAL 0x000000d1 00000000`00000014 00000000`00000002 00000000`00000000 fffff801`1bf951ee Ndu.sys Ndu.sys+41ee x64 ntoskrnl.exe+153fa0 C:\Windows\Minidump\070514-47750-01.dmp 4 15 9600 280.264 05.07.2014 22:12:27

Spoiler

070114-32828-01.dmp 01.07.2014 22:20:53 MEMORY_MANAGEMENT 0x0000001a 00000000`00003452 00000031`3dea1000 fffff580`10807748 e8500001`06c23c66 ntoskrnl.exe ntoskrnl.exe+153fa0 x64 ntoskrnl.exe+153fa0 C:\Windows\Minidump\070114-32828-01.dmp 4 15 9600 262.144 01.07.2014 22:23:36

Spoiler

062814-19765-01.dmp 28.06.2014 13:37:17 PFN_LIST_CORRUPT 0x0000004e 00000000`00000099 00000000`00155c51 00000000`00000002 0c000170`0017544f ntoskrnl.exe ntoskrnl.exe+1500a0 x64 ntoskrnl.exe+1500a0 C:\Windows\Minidump\062814-19765-01.dmp 4 15 9600 280.488 28.06.2014 13:43:22

Spoiler

062814-22218-01.dmp 28.06.2014 13:26:15 PAGE_FAULT_IN_NONPAGED_AREA 0x00000050 ffffd000`25ac93e6 00000000`00000001 fffff960`0014ca59 00000000`00000000 ntoskrnl.exe ntoskrnl.exe+1500a0 x64 ntoskrnl.exe+1500a0 C:\Windows\Minidump\062814-22218-01.dmp 4 15 9600 280.488 28.06.2014 13:27:34

Spoiler

062814-20765-01.dmp 28.06.2014 13:21:32 0x00000139 00000000`00000003 ffffd000`247e6a20 ffffd000`247e6978 00000000`00000000 ntoskrnl.exe ntoskrnl.exe+1500a0 x64 ntoskrnl.exe+1500a0 C:\Windows\Minidump\062814-20765-01.dmp 4 15 9600 280.488 28.06.2014 13:25:22

Just tell me if you need information from one of the dumps.

In all cases ntoskrnl.exe is involved, is this some indicator?
I'm not overclocking and run BIOS on default (as far as I know).
The processor is an AMD Phenom II X4 955 running on an ASUS M4A87TD EVO board.

I appreciate all help. Thanks in advance.

Regards
iBaer

EDIT: Just an additional note, maybe it's caused by the same thing:
Sometimes, when tabbing from one program to another the PC seems to freeze for a few seconds.
Also in most games I get the same problem at unregular times. The game freezes for 1-2 second, works for half a second and freezes again for about 4-5 times. Something that could be caused by my new graphic card (blue screens happened also before that).
Another thing could be my quite old hard drive (160 GB non SSD, it's 7 or so years old).

 

[MostWantedSoulRider]

Have you refreshed the PC yet? Windows 8 has provided the option to remove these kind of issues.

http://www.youtube.com/watch?v=YNwwC-cSYMg

Here's the link to see how to refresh your pc and it just takes few minutes.

 

[iBaer]

Actually I had to do 2 clean installs in a row, because the first one got corrupted due to an update. I tried to fix this with the refresh option, but it just made it worse, hence I made a new clean install. The blue screens appeared almost right after I made a clean install (the first 3 blue screens are "day one" bluescreens).
Do you still think a refresh could help with it?