[oneeyedpigeon]

Still on Mavericks. I didn't know anything about /var/log/wifi.log so I'll check that later to see if I can spot a problem - thanks.

[Fnoord]

Did you try the solutions at [1]? Have you considered upgrading? Be careful you don't get into issues with DNS related to [2] in 10.10

[1] http://howtoapple.com/mavericks-wifi-issues-fix/

[2] http://arstechnica.com/apple/2015/01/why-dns-in-os-x-10-10-i...