Problem w/MKV in Vista MCE not playing in widescreen

I just re-built my HTPC and I'm having some issues playing MKV files. When I launch the MKV it starts up scrunched into almost to a 4:3 aspect ratio, if I restore or minimize MCE it forces it to the correct aspect and plays fine from there. This is only happening with MKV files and it happens to all MKV files regardless of their aspect. My HTPC is configured as follows:

Hardware

AMD64 X2 6000+

ATI 2600HD Pro 512MB (HDMI dongle)

4GB RAM

Panasonic TH-58Z700U

Software/OS/Codecs

Vista Ultimate w/My Movies 2.42

PowerDVD 7.3(Ultra) lastest Service Pack

Haali Media Splitter - 1.7.401.3

ATI Catalyst 07.12 (lastest build)

Spdifer 0.2a

No other software installed


I've installed FFDShow via the Vista Codec Pack and it resolved this issue, but 1080p/DTS playback wasn't as smooth and would cut in/out loose audio sync etc. I would prefer to keep the existing software config as it's the smoothest I've found so far. Any suggestions would be appreciated.