Blender hangs in edge select mode.

With 2.78 just around the corner, I thaught I had better mention this here in case anyone else can shed light on it.
I am using the latest build bot build every day. cf53389 at the moment. For the last two or three days, I have been getting hangs in the 3d view port. 5 times so far. Cant reproduce it at will, hence no bug report. It only seems to happen in “edge select mode”. The 3d view port locks up, but by taking the mouse to another window. I have been using the header bar, and toggling a button, it dosent seem to matter which one, it then frees up the 3d view port again.

Im using the win 64 build with a 750 ti. 361.91 drivers

Naturaly if I can narrow it down any further I will post a proper bug report.