Unless you've got quite an old computer (with limited processing power) or there's a really 'heavy duty' program already running on your PC, the problem won't be related to the way in which your PC handles the data which is streamed to it.
That leaves only one other possibility, which is that your broadband connection isn't really up to the job. Run a ping test here. (It's not difficult; you just click a button):
http://www.pingtest.net/
You're looking for a grade 'A' or 'B' result, which is what you really require from your phone line in order to get reliable video streaming. (Click on 'Learn more about grades', from the results page, for a more detailed explanation).
The run a speed test here:
http://www.speedtest.net/
iPlayer can theoretically be used at a speed of 0.5Mbps but, for reliability, I'd hope to see a speed of at least 2Mbps shown as the test result there.
If the figures don't seem good enough, ask your ISP why you're getting lousy results.
In the meantime, clicking the link at the bottom of the iPlayer page which says 'Use lower bandwidth version' will probably solve your problem, but with a slight loss of quality in the image.
Chris