Consider the following setup: 2-3 Laptops and 4-5 Desktop PCs (both windows and linux systems) are connected to an ADSL modem/router (http://avm.de/produkte/fritzbox/fritzbox-3272/) via wireless and LAN. The LAN network consists of 2-3 switches of questionable quality as well as lan cables in awful conditions.
Now everything worked fine until someday the internet connections stopped working with limited connectivity errors in windows. I did some of the usual router on/off cable in/out tests and everything worked fine again. Next day I got a call again, internet not working. This repeats for like 5 times and right now I figured the following pattern: If I plug a device directly (no switch inbetween, no wifi) in the router lan port, connections work instantly and fine. If I unplug the device then, wifi and LAN via switches works directly after. But then, after some time, limited connectivity again. This is so hard to fix since I get everything to work, leave, and the problems occur sometime after that.
My main question is, is this certainly a router problem, or is it possible that some device in the network (broken cable/switch, infected pc or whatnot) causes routing issues for the entire network?