The wireless bandwidth is shared across all devices on that wireless band. That's why you're seeing such low bitrates when communicating between the PC and PS3.
Follow the FAQ for stuttering - viewtopic.php?f=6&t=3507&p=41214#p42818
As noted in the FAQ, wired connections are best, and if you can only wire either the PC or the PS3, then wire the PS3.
If you need to stick with wireless, try isolating the PC and PS3 to different bands (if you have a dualband router).