Latency problems on external interface

[tsdffl]

I'm having problems with my gentoo based router.

After being under fairly network load latency goes through the roof, the external interface showing ping errors:

An example:

64 octets from 64.58.79.230: icmp_seq=6 ttl=243 time=10084.9 ms
wrong data byte #0 should be 0xdb but was 0xd1d1 58 7a 3e e8 ec 0 0
8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27
28 29 2a 2b 2c 2d 2e 2f
64 octets from 64.58.79.230: icmp_seq=7 ttl=243 time=9575.7 ms
wrong data byte #0 should be 0xdb but was 0xd2d2 58 7a 3e 99 fe 0 0
8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27
28 29 2a 2b 2c 2d 2e 2f

--- yahoo.com ping statistics ---
220 packets transmitted, 218 packets received, 0% packet loss
round-trip min/avg/max = 1781.3/6537.7/11598.7 ms

I can't for the life of me figure out what is causing this. The problem only ever occurs on the external interface, the internal interface works fine.

I've switched round network cards (from 8139too to ne2k-pci) , and even run the same HD image on a different PC, but still the same problems.

The net connection is via a cable modem, which seems to be working fine when connected directly to a desktop PC.

If anyone has any ideas what might be causing this, or any ways to resolve the problem, then I'd be very grateful.


Router setup:

vanilla-sources 2.4.20 (have also used gentoo-2.4.19)
eth1 : Realtek 8139
eth0 : Realtek 8139
iptables 1.27a-r3
firewall script from projectfiles.com/firewall/
built from stage3 486 tarball