Have you got Java Script enabled and are you running the up to date version of Flash Player? Has the driver for your graphics card been updated too?
Have you looked at the resolution settings of your screen to see if that makes any difference?
There are quite a few problems for the Beeb to iron out with the new iPlayer and everyone seems to be waiting for a new software update - I believe it is still in Beta format. Even if your connection is fast enough, your ISP probably gives some types of data preferential treatment, particularly during peak times.
I have noticed since the new iPlayer was launched that programmes take longer to start and running the iPlayer seems to reduce the bandwidth to the point of it freezing and stuttering back to life and I have a very good graphics card. I had to alter the resolution on my screen to accommodate full screen mode.
Have you tried streaming iTV and 4oD players ? Do you get the same thing. Have you tried pressing the pause button for a while as the programme is loading. Have you tried using the lower bandwidth option? These won't cure the problems but they do sometimes help.
Another thing to try is perhaps another browser?