intermittant connectivity - MN-500

system - single MN-500
box 1 - p3 650, win98SE (used to set up the router
originally)
box 2 - athlon64, win2K Pro (recently added)
both connected via 100T ethernet, tcp/ip and netbui
protocols installed on both (wireless access disabled)

my internet and router connection drops out from time to
time, no apparent reason. time interval varies from
minutes to hours. internet connection seems to drop before
lan. modem shows no sign of disconnect.

currently trying to rule out isp as the cause (using a
public ip on the win2K box, with the router out of the
circuit).

any help would be greatly appreciated...i spent all day
trying to diagnose / fix this instead of gaming