There are other threads about wifi problems that you might want to search for.

What I did so far to be able to use SIP is to change my FB's (7270) wifi settings to
802.11b+g (disabling n) what helped a lot with lost connections. Both, ssh and
SIP are usable since then.

Concerning the ping latency, there was a hint to install wireless-tools
and turn off wlan0's power managment (iwconfig wlan0 power off).
This results in ping times of 1 ms but when the phone turns idle,
something turns on power management, again. So that is not really a solution...

Hope, that helps.