If you are sure about the modem upstairs being cable and not DSL, then you might be having signal degradation.
But from the sounds of it, your probably being capped by the service provider. I've heard of them capping DSL before but I never heard of cable internet being capped.
In the US at least, I've heard more of the opposite, that cable is more likely to be capped than DSL. It does sound like line degredation, but it could also mean that something somewhere on the network is failing, for example a dying NIC could be broadcasting gibberish and that slows the network down. If your on wireless, that in and of itself is a problem because it could be a whole lot of things.