The way it spreads and gets more and more frequent as well as the fact that it is still somewhat random and not tied to any specific sequence of events makes me believe it might be some kind of memory leak related problem on the server which keeps getting worse as it is up and running. If my suspicion is true, a simple reboot / restart of the server would "fix*" the problem for a good while.
*: of course, its not a real fix, that would require fixing the memory leak in the code, but a regular reboot would make the problem disappear from all practical aspects.