I have done some more testing.
If I start the kernel with the option "no-hlt" I get almost the same
improvement as when running a program that just consumes cpu.
I get some bit errors but most of the time uncorected errors is zero or very
low.
I had the exact same symptoms on my two Siemens DVB-C FF-cards (rev 1.5)
in two different cable networks and three different computers. Cards
worked usually fine some time, and then ber rose high and picture was
unwatchable, full cpu usage helped sometimes for a while.