There is a problem with the wireless (driver?) on the optimus and certain WiFi cards. I get 20-30% packet loss (which if you weren't looking out for, just looks like a very slow inconsistent connection) when connected to a Dell/Broadcom 1390 WiFi card for tethering. The packet loss is just pinging the phone itself and not going out on the Internet, so it wasn't the 3g connection affecting the test. When I connect using an Intel 2200BG right next to the Dell, there is no packet loss and the connection is pretty fast.
It is probably worth verifying if the inconsistency you were seeing was WiFi connection related of related to the tethering and Sprint.
In my case, it was the WiFi connection that was the issue with some devices but not others.