Games and 1080p movies started lagging

[smokesss]

Hello techies,

I reach out to you because I'm kind of a frustrated gamer . My PC went from running Crysis 2 on Ultra details in 1080p to lagging in Minecraft and preety much anything 3D about 2 months ago and it's been on like this ever since.

1. 1080p movies are laggy in any player except on Youtube, where I can even watch 1080p to 4k resolution videos ( the famous "Life in the garden" )
2. Games have a rather different symptom, they run smoothly for about 3 minutes and then one minute is laggy, the level of details doesn't matter, nor the resolution.

Here are my specs:

Mobo: Asus M4N68T-M LE
CPU: AMD Phenom II X4 Quad 970 Black Edition (BOX) - 3.5 Mhz if I'm not mistaken
Mems: ValueRAM DDR3, 2x4Gb, 1333 Mhz
Video Card: Sapphire Radeon HD 6870 1 Gb, GDDR5, 256 bit
PSU: Combat Power 550W
HDD: Desktop SpinPoint F3, 500 Gb, SATA II 300

All running Windows 7 Ultimate x64

Course of action already taken
- reinstalled Windows and ATI drivers
- analyzed HDD partitions and defragmented, it has a good reported status on several such utilities
- temps are regular, 49 in idle to 68 - 80 in-game (GPU with active cooler), haven't changed since I bought the PC
- BIOS updated, ATI Vision updated (Catalyst) & drivers (even went into the 12.7 beta version)

There are however several games which work beautifully (thank god). Games like Dead Island and League of Legends. About 10 other games that used to work don't work anymore.

Please Help

 

[Sunius]

Did you check temperature of CPU?

 

[smokesss]

CPU temps are also about the same, around 35 and 40 some in-game, they only go up once I unlock the fourth core which is locked via BIOS as an option to preserve energy consumption and deterioration.

 

[emesis]

I'd be looking at memory next. Run a memory test and see what happens.

 

[smokesss]

Memory test as in... running the Memtest application ? Should I also remove and reinstall the memory bars on the mobo ?

So... it can't be a physical malfunction related to the video card, if that were the case, the GPU wouldn't be working at all, right ?