Editor stuck on loading

I'm in the process of rebuilding my site in 7.1  but for the last couple of weeks I've had problems editing my site.

When I click the edit button it switches to the spinning square icon but stays permanently stuck at this point.

This happens less when I'm editing my home page for some reason and sometimes I can get through to edit my pages.

I've cleared the cache and am using chrome on my PC.

Any ideas - I'm losing hours trying to get around this!