This has to be the weirded bug.

I've got a  4801 that was running 1.3b9 - when I run mtr with a count 
greater than 32 it starts showing 100% packet loss at the 4th hop from 
the 32nd pass onwards.   0-31 are fine then bang it fails.   Always the 
4th hop, always the 32nd pass.   Even if I pick a different target which 
has a different 4th hop  it still fails at the 4th hop.

I uploaded mtr to the firewall and ran it there and it worked fine.   It 
only show up from clients on the lan side.

I downgraded to 1.233 and the problem went away.

Disabling traffic shaping didn't change anything.

To reproduce run "mtr --report -c 64 www.google.com" from a host on the 
lan side of a 1.3b9 firewall.

John