Problems with BBC iplayer the programme starts buffering and then plays the sound in sync and also simaltaneously out of sync, other iplayers work fine.

I have been watching the bbc iplayer and itv player without problems and then recently the bbc iplayer started having problems. The programme starts okay but then the litte circle of dots starts appearing and then the sound for programme plays twice once in sync once out of sync. The itv player is working fine. I noticed that when I perform a data speed check via my internet provider it registers around 20mbps when I use internet explorer but when I use Mozilla to run the same test it registers around 5-6mbps. I noticed that firfox updated from 4 to 5 but the problem does not correspond with this, I've updated adobe using the mozilla browser but this has not helped. I would really appreciate any help you can provide. Thanks