[Mick Fisher]

I would say your router can't handle the load.
Connect directly to your SACM to prove or disprove.
If so
Reduce the number of connections till it can or if that don't work beg steal or borrow another modem from somwhere.
In case you are running a java based client make sure you have the very latest java runtimes and turn off the auto update in the java applet in your control panel. It dont work and bogs down your PC through it's constant trying.
Always reduce your upload rate to 80% or less than it's theoretical maximum.

HTH