Audio issues

Hi,

I am running a Raspberry Pi model B with latest openelec 3.0.2 on SDHC Class 10.
My Pi is connected via HDMI to A/V AMP (Pioneer VSX 922) and this one is connected to a Panasonic Plasma.

The Media Files (most MKV files) are on smb share in my homenetwork, the Pi is connected via Ethernert Interface.
The Files are 720p or 1080p with AC3 or DTS @23,976Hz.

This issue I have is that the "Sound" of AC3 and DTS soundtracks are stuttering. Meaning that there is every few minutes a short "stop" in audio track or sometimes a "chopie/plop" sound. AC3 and DTS in the Settings is set to passthrough.

"Adjust display refresh rate to match Video" is set to "On start/stop"
"Pause during refresh rate change" "off"
"Sync Playback to Display" "off"

On the Pioneer Amp I have disabled anything regarding HDMI except Audio goes to AMP. (as described here)
http://forum.xbmc.org/showthread.php?tid=146911

I also have overclocked my Pi to 950MHz or tried another distro (Raspbmc) without success.
I still get the "stops" and "chopie noice" while playing a mkv file.

I also tried "Sync playback to Display" but as I already read this is not working with having audio passing through to an amp.
Maybe someone can help fix this issue?

Apreciate any help... hope someone can...

Thanks and Regards
Chris