NAT connection dropping

I have this box with 5 NICs working as proxy for my company with 300+
clients. eth3 is connected to LAN, and eth2 is connected to a switch which
also 3 broadband modems are also connected to that switch. Its very simple,
just use the following command for the default gateway:

ip route default replace nexthop via 192.168.200.7 dev eth2 nexthop
192.168.200.8 dev eth2 nexthop via 192.168.200.12 dev eth2

while 192.168.200.7, 192.168.200.8 and 192.168.200.12 are the IP addresses
of the 3 broadband modems.

It used to be working perfectly okay, until one day, connections will
suddenly reset throughout the whole company, those on web won't have any
problem, but all FTP, ICQ, YM... etc... connections will get dropped. The
funny thing is once the connection is drop, all client really move to
"nexthop", namely, if it was on 200.7, it would move to 200.8, and if it was
on 200.8, it would move to 200.12... etc.

Any pointer would be very much appreciated.

Thanks
Kenneth Ho

May I add that now the connections just drop every 10 to 30 mins which is
very annoying.

Thanks
Kenneth Ho