Ok, I think Timbo went to bed, but I've heard this comment a few times and I'd thought I'd offer some advice. SO, if you think you have a decent ping (average seems to be around 150) and your FPS is still fine, but things still seem all choppy and weird read on...
The default setting for 'rate' is 3000, which is supposedly good for dial-up connections, but seeing as how no one has dial-up any more, a higher number is needed.
Now, there are a couple ways a fixing this. Easiest is to join a game, and go into your options. Options->System->Net&Sound. Now, see the 'Net Data Rate' option? Set that to the appropriate connection you have. I recommend LAN/CABLE.
You can also go into the console (~ key) and type in "/rate 10000" (without the quotes). I've found this number to be quite adequate.