The TRACE definitely suggests a network routing problem. If you look at the output starting at
172.70.37.15 6.782ms asymm 6, those "asymm's are very suspicious. This page has an explanation of what they mean:
https://answers.launchpad.net/ubuntu...+question/7682
This is occurring before the network packets get near Fastmail's datacentre, and when I do a traceroute (or Windows TRACERT) it is clean in spite of doing this all the way from Thailand.