Trouble staying connected!!

[Tafkam]

I'm having this persistent problem and I hope someone may be able to suggest a troubleshooting starting point.
After I connect to the internet, after a few minutes, the connection just stops. The modem doesn't disconnect, but there's no online activity. I can't collect email, or browse etc. I have to disconnect (which incidentally takes anything up to 2 minutes, compared to instantly, if I'm connected properly) and reconnect, only to have it happen a gain in a few minutes. As it is, I'm copying everything I'm typing here, in case it happens right now!
I have 3 computers on a LAN, all net sharing from one modem, and while the problem rarely happens when only one computer is connected, it's inconsistent. ie: sometimes there's no problem when all the computers are online.
I'm trying to work out if
A) It's my ISP
B) ICS is overloaded with 3 PCs on a 56k modem
C) Some settings are wrong somewhere

Also....is the connection compromised if any of the computers are downloading stuff, or doing other resource hungry things?

All computers are running WinXP-pro
All file / printer sharing is working fine between computers.

I'm not sure what other info I should provide, but any suggestions would be very much appreciated. :)

[Styx]

You've answered your own guestion:

ICS is overloaded with 3 PCs on a 56k modem .

You need to be using DSL or cable to be using ICS. Plus your Network needs to be firewalled if it's not already. Scan for virus, trojan or variant. Sygate personal firewall is an excellent choice.