UPDATE: This issue has been fixed in Sketch 3.3.3. Go download the beta version now, or wait for the final version to be released in a few days.
The following content is kept here for historical reasons :)
We're currently investigating an issue where Sketch stops responding at regular intervals. Judging from our reports, it started about 10 days ago (June 1st) for no apparent reason that we can think of (our last Sketch update was released on May 11st). We have some hypothesis, but we need your help to fix it since we haven't been able to reproduce it on our test machines.
If you need a quick workaround, keep reading :)