Stuttering and low FPS when watching HDTV channels (1 Viewer)

[WebKnight]

MediaPortal Version: 1.1.0 BETA (Latest SVN)
MediaPortal Skin: Blue3Wide
Windows Version: WIN7 X64
CPU Type: AMD X2 4850e
HDD: Samsung 250GB + 1TB
Memory: HyperX 2048 MB (PC2-6400)
Motherboard: ASUS M3N78 PRO
Video Card: GeFORCE 8300 (Onboard)
Video Card Driver: Latest
Sound Card: Realtek HD Audio (OnBoard)
Sound Card AC3:
Sound Card Driver:
1. TV Card: Hauppauge PVR500MCE
1. TV Card Type: Analog
1. TV Card Driver:
2. TV Card: FireDTV
2. TV Card Type: DVB-C
2. TV Card Driver: 5_2
3. TV Card:
3. TV Card Type:
3. TV Card Driver:
4. TV Card:
4. TV Card Type:
4. TV Card Driver:
MPEG2 Video Codec: PowerDVD 9
MPEG2 Audio Codec: Latest SAF
h.264 Video Codec: PowerDVD 9
Satelite/CableTV Provider:
HTPC Case: Silverstone SST-LC17
Cooling: Stock AMD
Power Supply: Fortron/Source ATX-400PNF
Remote: MCE remote
TV: Samsung LE40M86 Full HD
TV - HTPC Connection: HDMI-HDMI

I get 16-18 fps when watching HDTV channels. have tried to record a show, and it's the same when trying to play the recording. But if I open the recording in windows mediaplayer outside MP it plays perfect. Tried PDVD9 and CoreAVC decoder, both with the same result. Have had this problem no matter witch SVN release I've used

Any ideas to what causes this?