Okay, it's been a while since I last played with the set-up (I actually put my Hauppauge PVR-350 back in the machine to have 3 analogue tuners...). But I bought another DVB-C card (a cheap used TerraTec Cinergy 1200 DVB-C), to see if the Twinhan card was responsible for the freezes in GB-PVR.
It turns out that it probably isn't a hardware fault, but rather NewCS that freezes the machine. I tried a new cardserver, ACamd, which isn't a stand alone program, but a multidec plugin. And voila, it works reliable!
The strange thing is that while I do get perfect live tv in GB-PVR (stable too :o)), I can't actually get it to record anything meaningful. The only setting I get files on is the TS (TransportStream) setting. But the files I get aren't unencrypted, so they're no good...
I'll have to spend some more time tinkering with it :)
GB-PVR Forums - Public - Support: DVB-C MD - Can't record (live tv works)