I suspect that there is a lot of broadcast traffic on this network. It’s not necessarily collisions but each machine has to listen to all broadcasts on the network. The broadcasts are likely routing updates from the DSL modems. See if you can disable routing updates on the LAN side of these modems. Your next option is to subnet the network into multiple subnets and use a managed switch as a backbone router rather than going to the individual DSL modems. Another thing to confirm is if DHCP is enabled on the DSL modems. This could confuse network devices pretty quickly if there are multiple DHCP servers on a LAN.
Can you also explain why you have 10 DSL connections rather than a single T1 or similar link? It seems like the 10 connections may be more expensive and require more management time than a single link for these users.