Have you checked the firmware for an update? Or have you thought about trying out DD-WRT out? http://www.dd-wrt.com/wiki/index.php/Linksys_WRT400N
As a hold over I decided to give dd-wrt a try, something I've contemplated a few times over the years but never actually got around to doing. Finally glad I did though, thanks for the reminder.
Following the steps via the link you provided to flash my wrt400n with dd-wrt has greatly improved the stability of the router to the point where I haven't had to reboot it in 2 weeks, this was previously something I was doing every couple of days.
I still don't have the gigabit support or ipv6 but those are items I can survive without for the moment. Thanks again!