"PUT user" request timeout

For the last few days we are encountering an issue when posting requests to CB server.
When sending a login request from our application and the login username is not registered in CB server, we do “PUT _user” request, but the response does not come in the following 5 minutes. We are using two buckets - one “main” and one for replication and documents history. Could not find anything in CB server log files. If we delete the replication bucket and recreate it, it goes OK for a few hours and then starts timing out again. The main suspect is that some of the documents we put are too big for CB, although we have set max size 100k. Could not find anything in the logs.

Using CB server 2.2.0 and Sync_gateway on different machines, both Win server 2012.

Any clue what could be the source of the issue or where can I search for it ?
Thanks in advance.

Sorry, forgot to mention a few more things. Erl.exe is taking as much CPU as it can take while this is happening. CB server service restart does not help. We are sending all requests to sync_gateway, not CB server itself.