It seems to be related to BBC Iplayer only. But they said...
However, a large number of non BBC factors may affect the performance of iPlayer, including;
Your PC`s performance.
Your Modem/router (wireless performance, distance from Laptop/PC, firmware, firewall, number of users etc).
The quality of your telephone line (ADSL – Noise levels, distance from exchange).
Your ISP`s infrastructure (router, servers, proxies, DNS, mail, QoS) and the size of its link to the Internet backbone and capacity management policies (throttling, excessive usage policies etc)
The number of customers using your ISP right now - contention ratio.
Load on the whole World Wide Web Servers and/or Internet links.
3rd party software in use - spyware, adware, security suites, antivirus, firewalls, media players, browser settings (flash/javascript/activeX activated, pop-ups acknowledged) and browser toolbar add-ons, peer-to-peer traffic & applications and any other 3rd party application(s) that may also be utilising your system resources.
Have you Tried any other site's that stream videos?
Hope it helps :)