The limit is set at 100 connections (on the old server it was 5 or 10...), at some time I was seeing the list of 100 query results staying in the queue for clients which asked these. Now the queue is empty despite the fact that there must be as many people on the site as few hours ago.
So I really think it is not a problem in imcdb but just on the server itself.
I made a small change to use persistent connections (mysql_pconnect instead of mysql_connect) just to see if it helps. But as currently it seems to work I can't know if that change will help in case it fails again