That doesn't make much sense. There shouldn't be any compatibility issue, as it's just 10/100 ethernet all around, and those problems were well sorted out by the time that router was released. I was going to ask if maybe the internet side of the router was 10Mbps only, saving Linksys 5¢, but the spec sheet says all ports are 10/100.

Is it possible that you have that port forced to a specific speed/duplex setting that now needs to be updated?

Can you try putting a switch between the modem and the router?