You are right--the client hang up before the server was able to send a response back to it. In this case, the server received the client's request, and client was waiting for a response from it. The length of time for which the client waits for a response from the server is set by the client's read-timeout setting.
Also, it's worth looking at why the server sometimes (not always) takes so long to respond. Could it be that some function invocations take longer? In that case, the client's read-timeout should be increased to account for such possibility. Or, could it be that sometimes something else goes on in the server that makes it unresponsive? Your previous issue I was just looking at comes to mind here--in that case the culprit was a GC pause. This issue could be caused by a GC pause as well.