Both of those answers are completely correct but they seem to be ignoring the actual cause of the problem, which is either a poor quality phone line or insufficient broadband capacity.
Almost all ISPs use some form of 'traffic management', which means that they 'throttle' download speeds for certain types of internet traffic and/or at certain times of day. Additionally you'll be effectively sharing your broadband connection with other users in your locality. 'Bargain basement' ISPs usually have high 'contention ratios' (so that, for example, you might be sharing a connection with 50 other users). Better (but more expensive) ISPs usually have a contention ratio of no more than 20.
So it's likely that your internet connection simply isn't good enough for reliable access for video streaming. Use the following two sites to check:
Go here to run a ping test:
http://www.pingtest.net/
That will effectively check the quality of your phone line. You're seeking an 'A' grade (or, at worst, a 'B'). If it's lower, get BT to check your phone line.
Then go here to run a speed test:
http://www.speedtest.net/
Run it at several different times of day. If you don't consistently get a speed of at least 2Mbps your ISP isn't providing a good enough service for video streaming.
Chris