IP masquerading lockups in 2.3.99pre3, 2.2.15pre13

My Linux firewall box has been very windoze-like lately:  it rarely
stays up longer than a day at a time, and under heavy use only an hour
or so at a time.  I am at wits end as to what the problem is.  Under
kernels 2.2.12-20 (the standard Redhat 6.1 kernel), 2.2.15pre13, and
2.3.99pre3, I have not been able to sustain an uptime beyond an hour or
so when I am running Quake3 behind my firewall.  Most of the time, the
firewall just locks up solid; no numlock response, no magic sysrq key,
nothing.  Other times I can ping the firewall for a few minutes but X
appears frozen, and I cannot ssh to the box.  There are no messages to
any logs, even though I have syslog logging facility.severity *.*.  Many
times the box has hung during the required fscks during reboot.  It has
gotten to the point where I mount /usr, /opt, and /boot read-only by
default to avoid most of the downtime upon reboot.

Given this tragic history, I started to strongly suspect hardware
failure.  So I downloaded the VA Linux Cerberus Test Control System
(ftp://ftp.varesearch.com/pub/software/Cerberus) and, ignoring the dire
warnings contained in the readme, ran their burn-in tests for 4.5 hours,
resulting in exactly 0 failed tests.  

My configuration:

Redhat 6.1.
AMD K6-3D 300
Microstar MS-5169 ATX AL9 motherboard
300W power supply
128MB memory
1 10GB IBM IDE (hda)
1 CDROM (hdd)
2 9.1GB Quantum Atlas IV (sda, sdb ganged together as a raid1 md0, only
on 2.2.12-20)
1 3c905 NIC
1 3c905B NIC
1 floppy
TNT2 Ultra AGP
SB16 PnP

Output of ver_linux:
-- Versions installed: (if some fields are empty or look
-- unusual then you possibly have very old versions)
Linux localhost.localdomain 2.2.12-20 #1 Mon Sep 27 10:25:54 EDT 1999
i586 unknown
Kernel modules         2.1.121
Gnu C                  egcs-2.91.66
Binutils               2.9.1.0.24
Linux C Library        libc-2.1.2.so
Dynamic Linker (ldd)   2.1.2
Procps                 2.0.4
Mount                  2.9u
Net-tools              (1999-04-20)
Kbd                    1999.03.02
Sh-utils              
2.0                                                                                           

Also:  ipchains 1.3.9, 17-Mar-1999

Does anyone have any ideas about what might be going wrong here?

Thanks,
Brian Strand