Perhaps it's the same problem as I have with my unmodded FF-Card
Same here on vdr-1.7.12 with reel snap -r14440 ( starts on update to vdr-1.7.11 ) terractec-1200 dvb-c card latest vanilla kernel sources.
this is on all sky channels with resolution 1920 x 1080
If i go down to vdr-1.7.10, all problems are gone.