(1) Make sure your flash player is up to date
(2) Make sure your web browser is up to date
(3) Check you've enabled HD mode in iPlayer
(4) Make sure your mouse doesn't have focus on the controls (i.e. move your cursor somewhere random in the middle of the screen)
(5) Don't waste your money on a "wifi extender" .... pure sales snakeoil. If you can't get a good signal where you are, then a "wifi extender" aint gonna help you one iota unless you are planning to run a CAT5 cable between your router and the extender.