I have been fighting with the very same issue since I upgraded my system with a new MOBO (Asus A8V-Deluxe) and an AMD X2 5800+ CPU. Same graphics board (Asus N6600TD) and same OS (Ubuntu 7.10 32-bit) brought to an infinite series of random freezes. Total lockup, no response at all from magic sys-rq keys or anything else. I filed a bug report to nVnews, which remained unanswered.
After a LOT of struggle, thinking it was an issue about interrupt handling like the thread starter noticed, I found that reducing the "aggressiveness" of interrupt rescheduling could effectively reduce the freeze frequency. So I introduced this command in my /etc/rc.local:
echo 1 | tee /sys/devices/system/cpu/sched_mc_power_savings
This is a feature discussed here:
Looks like the interrupt switching between cores could be responsible for the freezes. Of course, this only happens with multicore systems, and in almost-idle conditions (just like all the freezes I had, always happening when I resized/focused a window). Compiz shouldn't be liable, because I tried deactivating it for a while and still had freezes.