I had similar symptoms i checked all may software (configs ) and nothing found.
So I compile new kernel 18.104.22.168 with
AMD74xx -> for may Nforce 2 Ultra 400
AGP and Nvidia -> for AGP support
NVram and MTTR -> for driver
and i have the same result so i start check hardware
then i found core of the problem may Power Supply was too weak or damaged because on Vcore i was have 1,2 V - i check with voltmeter - you can use lm-sensors tools for check but if you use voltmeter you have more near to truth - check processor in streets and check Vcore Voltage.
I was have ModeCom Feel 300V - one big **** - now I have Chiftec 450 V with AFPC and other ... - I think this may case because your power Supply is too week.
may configuration :
ASROCK K7NF2-RAID (nforce2 Ultra 400)
Athlon XP 2200+
ASUS 6200N 128 MB DDR2
2 optical drivers
2 hard disks
1 GB DDR2 333
Chiftec 450 ( Power Supply)
Ubuntu 6.10 (with last update)
Nvidia driver 1.0-9755
kernel 22.214.171.124 <- recompilation for may hardware - building from source debs