From what I can tell, your internet connection is dropping - correct? It
works for awhile and then stops?  When your connection drops, is your
modem online?  Look at the LED's on your modem when the connection is
down, and see what the manual says about the modem's status at that
point.  Your modem's log indicates that it is loosing sync with the CMTS
- nothing m0n0 or any other computer can do to cause or fix that.  That
said, there are a few things to try.

1)  The ICMP packets are just the DHCP server pinging the address to see
if it's available - no worries there.  However, you are spoofing a MAC
address on the WAN.  The problems you are seeing could happen if someone
else has statically assigned the IP address to their PC/Router that is
being handed out to you by the DHCP server.  If Comcast is doing things
right, they stop this, but many cable providers don't.  The simplest
solution if this is the problem?  Spoof the mac address of the XP
machine - this will give you the same IP as your XP machine had - verify
it to make sure.

2)  You mention storms.  Many times, water can get into the splices
between the pedestal and your modem, which causes a lot of ingress. 
What are the signal levels on the modem?  I'm 99% sure that the problem
is with your signals, as those T3 time-outs are warnings about a loss of
signal from the coax port on your modem to the upstream port on the
CMTS.  Log the time from which your connection drops, and the times that
it comes back.  Compare those times to the T3 timeouts (modem goes down)
and the "registration complete" messages on the modem.  If they match
up, call Comcast and have them dig into why your modem is going offline
so much.

Let me know,

Justin

-- 
Justin Ellison <justin at techadvise dot com>