I created this new task because previous ones have been mixing up intermittent 503s and permanent ones caused by other issues. This task is only for the intermittent errors that occur.
It's reported many times by several users that sometimes when they try to access a page on their wiki, they get a 503 error which lasts for a short period of time. Per T1979#42376 it could be something with Varnish, since even after mw2 was upgraded, the problem is still persisting.
Even though this doesn't cause downtime exactly, it is still inconvenient for users and should be looked into.