Yup - you're using a Linksys WRT54GS, right? We have some discussion about that problem here:

http://www.nessoft.com/forums/showflat.php?Number=1585

Summary: download the latest bios for your router. If that doesn't completely solve the problem, then change PingPlotter to start tracing at hop 3.

Please let us know if that doesn't get things working for you.

- Pete