STUN fails on asterisk-1.6.0-beta9

My asterisk-1.6.0-beta9, configured with, runs on my LaFonera WiFi router with OpenWRT SVN firmware has been spitting out the following error messages from STUN:

Has anyone who uses asterisk-1.6.0-beta9 encountered such a problem? Is there a more reliable public STUN server out there I can try to resolve this issue?

I noticed the above error messages if I configured my asterisk-1.6 with more than 5 channels. Does this mean the STUN function sends out STUN request/channel to a STUN server? If so, how does a STUN server treat N requests from the same IP Address if the asterisk is configured with N channels? Will the STUN server consider those N requests from the same IP Address as a flooding attack and responds with DOS?