I'm having a strange problem on a connection behind a Cisco modem connecting to a DSL network : outbound connections hang after transferring exactly 1024 bytes. Inbound connections are just fine. This is 100% reproducible on all of the router ports and on all computers on the network.
Not being a network kind of guy, I can deduce the modem/router is at fault, but which setting could cause this behavior ?
Try lowering the MTU to 1492 or even a bit lower. You could keep lowering it by 50 or 100 (until you get to 1000), to see if it will start working more reliably at some point. If you go too much lower than 1000, than this is not the issue.