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So your router appears to be fine, normal download speeds etc, with just one PC, but not if you connect more than one PC?
Do your PCs have current antivirus and the like. Do you have anythign running that might be drawing a lot of bandwidth up or down, that you know about, or don't know about?
Is your connection reliable when you only have one PC connected directly to the modem (no router).
Answers to all of these questions will help to determine if it is broadband (unlikely), or something about your router (more likely) that has gone wrong.