We have uploaded a new 4.0.10 build, it should be the final build.
Have been tested on a few busy site that experience 503 errors, the latest build is very stable. So, all user who have manually upgrade to 4.0.10 release should download again and do a manual upgrade again.
Doing so now but backing up the current 4.0.10 build we have as we've not had any issues with it
Hopefully Dan from EZPZ will report this has resolved his issues - any insight as to what was causing the 503s (if you can post it)?