linksys wmp55ag difficulties

I have a Linksys wrt55ag wireless router, and two PCs with wmp55ag nics. One
PC is running XP home SP2, one with Win 98SE. Lost connectivity with PC
running Win98 after about 2 months, and after upgrading it to W2k SP4 and
about 3 hours on the phone with linksys (normal troubleshooting type work -
uninstall, reinstall, reseat nic, different slots, different channels, move
closer to router), linksys went for a replacement on nic. I have
re-installed nic, and have been able to connect and get to internet for
about 3-4 minutes several times. After 3-4 minutes, I lose connectivity to
router, and have to uninstall/reinstall nic software to get it back - only
for connectivity to last for 3-4 minutes again.This router/nic combo allows
for 802.11a and/or 802.11g - I get same exact results on either a or g.
Linksys software always reports an excellent signal with access point
(router), but can't see internet.

When I am unable to get to internet, ipconfig reports proper ip
configuration: gateway = 192.168.1.1, dns seems to have proper entries for
ISP, proper IP address from DHCP, etc. But I can't ping router or any other
IP address. Always happens after a few minutes of working fine. Resetting
router/cable modem makes no difference.

I have tried resetting RTS & fragmentation threshold, beacon interval per
Linksys help suggestions, have disabled WEP The 'new' nic is an older model
wmp55ag, and appears to be a remanufactured one. Since I have never lost or
experienced any problem with connection on XP pc, I suspect the router is
functioning perfectly normally, and since the PC worked for about 2 months
before, I am inclined to think I may have another bad nic -but I have worked
too long with PCs to rule anything out. I have e-mailed Linksys about this,
and haven't heard anything back from them.

Interesting puzzle, but does anyone have any ideas?

-Thanks.