@Sp1derman:
Crap... eth0 is onboard.. I can't change it. (I was sure it was intel!)
I do have an intel NIC around, too, but it does not have a low-profile bracket. And adding an NIC invalidates my license.
The tests:
At this point I narrowed down the disconnections to the work-issued VPN. Booting the second system no longer kicks me off the connection if the VPN is not running on my system. The log gives no messages when that occurs. Booting my system when the other one is up and online no longer kicks it off of it either. That might have been due to the bad 1st install or a bad setting somewhere that wasn't default or both, on TOP of having my VPN running on work days
I can't think of what I can do to solve the VPN issue as I doubt the I.T. people at work would have a solution too.
I got more things to try out before going that route.
Looks like the default setting is what works best.
Things have currently stabilized and perform as intended, so long as I don't have to use the VPN.
It doesn't hurt that the modem has been changed as well for a small upgrade to the connection.
Right now I'm keeping a close eye on the telnet feed from the router and it's all looking good, too.
So it seems that my woes were a mix of so many things that there was not a definitive solution to it. (Bad install, config changes done too fast/too many times?, modem that never worked well, VPN...)
Thanks again for all the advice and help. It's a huge plus to know how my system works
With or without VPN, there are no disconnects anymore.
ISP provided an upgrade to my connection at not extra cost which included a new modem.
I still require the router to distribute the signal and the computers don't kick one another off the internet anymore.
It seems my suspicion of the modem being the root of all the troubles was correct after all. I have no idea why, but it was.
I haven't fiddled with the DD-WRT settings more than I needed to and since things are working full-speed at near-default settings.
Hopefully no one else here has to go through that weird rigamarole of issues with the same type of cable modem and with any luck this experience can help them.