Odd ICMP packets and memory leak

First off, thanks to those who replied re: Happy Medium (linux/windows
co-existance), I guess an A/B switcher is the way to go for now. ;)

I have been trying to track down a memory leak our server has had for the
last few days. It started Friday when the server would crash (more
accurately the kernel would kick out user-space programs because of 100%
memory usage - 1 GB physical and 1.9 GB swap -- overkill, but this was an
LTSP server at one point) every 2 hours or so. I figured out this weekend
that the problem is only present when the network is up and our iptables
nat script is run. I then used iptraf to figure out what systems might be
causing problems and found that 2 systems were sending out ICMP packets.
One was infected with the Welchia worm which has been taken care of and
the other didn't find any viruses (Using Norton with latest definitions).
I am trying to pin-point this problem, and am thinking about just blocking
all ICMP packets that are outbound over NAT as a temporary fix. I'm not
going to be in the office until Wednesday and need to get this up and
running tomorrow morning if possible. Here is what my iptraf monitor
shows:
ICMP echo req (92 bytes) from 192.168.0.146 to 192.166.55.110 on eth0
ICMP echo req (92 bytes) from 208.186.160.162 to 192.166.55.110 on eth1

It repeats this one right after another, with various different
destinations, all in the 192.x.x.x network.

I have tried various commands such as iptables -A INPUT -p ICMP -i eth0 -j
DROP, but none have blocked this. Am I missing something obvious?

Thanks.

Jason Pfingstmann