Does this mean you loose packets from the OpenWRT router to the ENC28J60 ethernet shield/breakout on the Arduino? If it's on the WiFi part, it's not Arduino related.
BTW, following you description of the path from the joystick to the bot there are many, many possibilities for a lost command. As far as I understand you even don't know if you're loosing packets on the network or if just the command get lost somewhere on his way to the bot. Correct?