Unfortunately, it still took me far too long to figure out why that was. First, I was suspecting interrupts, as the nvidia driver and second of my two DVB capture cards were sharing the same interrupt line. After experimentation with kernel parameters like acpi=noirq, apic, ioapic, lapic etc I finally did find a configuration that put the nvidia interrupt on its own APIC IRQ line. However, that did not help.

What finally did help was forcing the AGP rate down from 8x to 4x. This would probably have been the first thing I would have tried, except that my BIOS has no such option, and in the NVIDIA closed-source driver it is disabled by default. I had to resort to rebuilding the kernel module to enable that option. Why is that? Every other display driver, whether with a kernel module (fglrx) or just an Xorg driver, has enabled a manual setting for AGP rate without having to resort to source code modifications. Highly annoying.

Good news is that the problem is finally eliminated, and our living room entertainment system is once again rock solid. I've woved to never upgrade the kernel and nvidia driver on it again after this experience..