I mentioned this in an Exchange / CAS thread, but will mention here as it may apply as well.
We spent about a year troubleshooting slowness issues with Exchange, and getting weird performance / errors that didn't make sense.
After working for a LONG time with F5 on the issue, we discovered that the Nagles Algorithm was the culprit. Google it for more specifics, but it basically combines smaller packets to send more optimized larger ones on the back end. This is great for general web traffic, but not necessarily so good for RPC related traffic. RPC expects immediate response, and doesn't really like packet alterations.
You may try with it disabled to see if this helps.