Thanks for your help, it's interesting input!
I can somewhat follow the problem and I think it's related to the memory leak issues, which were reported.
There are a lot of unclosed connections/sessions, which result in leak and in your problem.
It is said, that the memory leaks don't occur in 3.9.1 (at least they are not that severe).
Maybe you can try 3.9.1 and report back, if the problem is gone. That way, we could narrow it down to be a 3.9.2 problem and that both problems have the same cause.