Geoserver overload

we've been using Geoserver v2.2.5 for quite a while and we've started experiencing difficulties recently. Currently we have about 7 000 layers published in Geoserver served via WMS.

What happens is we get "exceeded memory buffer" error when accessing the map. It seems to happen randomly, unrelated to any of the published layers. What bothers me is that the problem occurs with very small number of users using the map (actually we are able to overload Geoserver with 4 concurrent PCs accessing its layers). The request is sent to Geoserver, but it is never finished and hangs somewhere between the server and Geoserver. It all ends up with Geoserver getting stuck and unresponsive (needs to be shutdown manually).

We've tried rising timeout for requests (did not help), switching from OpenJDK to Oracle Java (did not help).

I've run out of ideas. Has any of you ever experienced anything like this? I'm looking for solution as well as for your user experience: how many layers is published on your Geoserver, how many people access them?

Thanks for any help.