I will have to check if it occurs also during playback.
Where to I set and increase the SDRAM value? Is this the ammount of shared RAM, reserved for the GPU?
you need to add the line
to config.txt. see here for instructions.
Reboot pi. Then SSH into pi and run the vgencmd command above. You need to run the vgencmd after ever reboot.
Try turning off deinterlace first in video settings before changing sdram see if it stops the artifacts. If not then try the sdram & vgencmd.