TVHeadend + xbmc strong artifacts

Hello,
i have a big problem with my tv setup. I have a tv server running on ubuntu server (12.04 LTS) with tvheadend. The clients use xbmc. But on every channel I get strong artifacts. I already deactivated the idle scan in tvheadend but it didn't help. I think it has something to do with bandwidth or buffer size, because on SD channels the artifacts aren't as strong as on HD channels.
I already tried the card on Windows and there are no artifacts (same softwares used), so it hast do be something with the drivers or linux itself. Setting pci-e speed manually to 100 Mhz didn't help and my bios does have the newest version. All i can say so far is that tvheadend trows out discontinue errors every time artifacts are visible.
I have also attached a screenshot, maybe it might be helpful. (its not always that green, its only the worst case. Most of the artifacts are just some wrong blocks)

i have a big problem with my tv setup. I have a tv server running on ubuntu server (12.04 LTS) with tvheadend. The clients use xbmc. But on every channel I get strong artifacts. I already deactivated the idle scan in tvheadend but it didn't help. I think it has something to do with bandwidth or buffer size, because on SD channels the artifacts aren't as strong as on HD channels.
I already tried the card on Windows and there are no artifacts (same softwares used), so it hast do be something with the drivers or linux itself. Setting pci-e speed manually to 100 Mhz didn't help and my bios does have the newest version. All i can say so far is that tvheadend trows out discontinue errors every time artifacts are visible.
I have also attached a screenshot, maybe it might be helpful. (its not always that green, its only the worst case. Most of the artifacts are just some wrong blocks)
