Well, unless there's a hard drop in connectivity, it sounds as if there is a collision issue in your network, which could be random, it could be actual physical wiring/cabling.
Because you're talking about (it appears) an intermittent drop, and other elements of your infrastructure are still online...I would lean towards a "quick" check of the physical connections and cabling. If you can isolate the drop via a tracert to a specific switch or port group, that may narrow the search.