Some questions that need to be answered before being able to help:
1) Can you update to the latest beta (1.2b10)? I doubt that will fix
anything, but 1.2b10 is rock-solid, and you'll be running the latest
2) Can you max out the link with the traffic shaper disabled?
3) What NICs do you have in the machine?
4) Are you forcing the speed and duplex on those NICs?
5) What are the CPU and memory usage at times when you try to max out
6) Any errors in the logs?
7) Goto exec.php, run 'netstat -i' and post the results.
If you can get us more info, we should be able to get you fixed up!