to Cable Forum
If you have a network card, swapping over to it rather than using the USB can be more reliable, simply as less of the main computer resources are used. However that may not be the issue here.
It's also worth checking your system is free of spyware and the like. And of course you are not using any P2P type software, all of which might be creating upload use, saturating the bandwidth.
Most likely though something has gone wrong with the STB, or the cable connection, especially if it used to be alright. When you are near your computer / STB, call faults. They should be able to check, and if needed can send an engineer to adjust the connection.