I have 3 boxes using the internet (1 Scientific Linux, 2x XP boxes).
The XP boxes have a Sygate firewall running constantly.

We have a Dynamode (cheapo) 8 port hub model 80010-D fed into a Zyxel prestige 600 modem configured which we see as a DHCP server. It's set up beside my box and I can see the leds on both devices

It all works, but access can be very slow - mailchecks time out, pages take a long time even when no network load exists. Usually the router is blinking slowly, but when the problem exists, the router (1 input and the output) blink rapidly, and nothing goes in or out. It's not always the same box. I have linked it with firefox on the linux box, but it doesn't go when I kill firefox - I must reboot. We have had it just now and the XP box playing counter-strike (low KB useage) halted everything.

I know I can forward through my box, but it's not left on 24/7
Any advice please, on faultfinding this, or cures?

Thanks for reading this far!

Well, if you Windoze box has the problem as well, this is obliviously a problem with the router. Some router are badly made and may crash if they are getting too much trafic, it's not uncommon that the total of bandwidth a router can have exceed it's cpu capacity (making it acts a slug). If it's your case, I fear there is nothing you can do.
Try to check if the same problem happens with another router (borrow one to a friend?), in which case you would know that your router is buggy.
Try to update it's firmware first, it could fix the bug, who know?
If you are still under warranty, you might want to return this one to the vendor.

Note :
I don't quite understand where is your router here however, you are talking about a hub, a modem and a router but you didn't tell us where the router is exactly, I assume it's the "modem" you were talking about.

Sorry for being unclear.
No router - the hub takes the three boxes - excuse my typo.
The Zyxel Prestige mdems are the 600 series - but it's actually a 623R. Plenty on the web about that. It offers a configurable dhcp server with configurable NAT. It offers a 33 node dhcp server and assigns IPs by nic hardware addresses (The 8 digit hex identifier in the nic).

It's the fact that there's low/no network traffic when it jams up that galls me. Yesterday I couldn't receive or send mail (Timiing out trying to reach an existing server) while the only other box on was jamming things. That box's firewall showed 10k/s in, and 2k/s out (Gaming) on a 512/128 link. Gaming is normally never noticed. Both my linux box and the xp boxes can jam the network. I don't get it.