Audio fades and dies

Hello,
(this is my first post in here so sorry for 'lack of knowledge' and 'crappy english')

Since two days I have a problem with my audio...
It started after installing codemasters Dirt3.
I also installed F1 2011 after that, and know they both come with 'rapture 3d'

Problem however is that even when using 'software'-setting instead of rapture,
the sound is good for a few minutes then fades to left-speaker for another few minutes and then it's completely gone.
This only happens when playing games (playing songs, watching movies, audio on f.e. youtube or other internet-apps gives no audio-problems)

The strange thing is that I already had Codemasters Grid installed, and played it for a few months without any problem. Now the audio-problem also appears when playing Grid.
Also after the problem has occured, the sound stays gone for a few minutes (so no sound on other then gaming-applications)

Specs of PC:

ASUS P7H55D-M Pro motherboard
Intel i5-650 processor
using build in HD audio (realtek ALC892 codec)
win 7 Ultimate
speakers: two creative speakers + subwoofer (on 1 jack)


Things I already tried:
- changing Rapture 3D-settings
- set audio to software-driver instead of rapture
- uninstalling rapture
- uninstalling realtek audio-driver and re-installing (from asus site)
- changing speaker settings on realtek-app (from plain stereo to 5.1,...)
- puting speaker jack from back- to front-input

Really getting desperate because it's so frustrating to have good sound first few minutes, only to discover that it still fades after 10 minutes, and thus have to exit game again :(

Hope someone can help me with this one
 
Yesterday I firstly 'undusted my motherboard, graphic card,...', because saw that temperature in PC was getting very high
(suspect that this was the main cause?)
then went by 'method of determination?':
- deleted Dirt 3, F1 2011 and rapture from PC...
- tried audio in Grid (no audio problem)
- installed F1 2011 + rapture (no audio problem)
- installed Dirt 3 (no audio problem)

Played Dirt 3 and F1 for more then an hour without fading sound,
so hopefully the issue won't come back
 
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