That's not good when you notice the drop in speed is your computer connected wirelessly or wired? If it's wireless it could be a problem with the wireless channel and resetting the hub is changing this for you. Check out this link and let me know if this helps.
thanks spoke to BT chat and they also sugested same thing. Have manually set the channel and am using a WIFI analyser app that they suggested to make sure the channel is clear. amd experiencing the problem less now, so hopefully that is the fix.