"Connection Reset" with Standard Virtual Server

First, I would use auto map and instead I would setup a SNAT pool with the virtual server IP in question and use that as the SNAT pool in order to avoid possible issues with high connections. Second, what happens when you follow the tcp stream on that 400 bad request? Are you sure the client isn't making the request? You might be able to follow the ephemeral port for the request to the pool member because typically the 
F5 attempts to use the same ephemeral port that the client uses when SNAT has been enabled.