Harald,
if - the problem surfaces only with a new DVBs hardware card (and not with the old one) and - it only appears when you have traffic both on the DVB card and on the network interface and - the problem only goes away when you cold boot
then this makes me think that you have a problem with the PCIe bus.
I trust you have updated the mainboard BIOS to the latest level, right ?
Can you please elaborate on the hardware * mainboard make and model * schematic of the mainboard and how network card, disk controller and DVB card are attached
Please run an iperf test (bi-directional) to measure the network performance when newly booted and when "the problem" has surfaced.
And I would guess that you can trigger the problem more easily if you run iperf, create disk activity (run a random access read-only disk test ?) and run the remote video client at the same time.
Btw, there are LEDs on the Digital Devices card, do they change between the "good" and the "bad" state ?
Thomas