You can never track down problems like these unless you can see what is going on - you need monitoring and visibility to see what your devices and internet link are doing
Firstly, set up something to monitor the utilization/bandwidth on the internet link itself. If you want free tools, try MRTG, http://oss.oetiker.ch/mrtg/ or http://www.mrtg.com
Once you've got that, you can see if your link capacity is being hosed but you can't tell who is causing the problem. To find the culprits try implementing one of the many SNMP/RMON monitoring tools that are available - again, some are free. Set it up to monitor your router, switch or another suitable interface through which all your internet traffic passes. The features you are interested in will be things like Top Talkers (who is using the most bandwidth), Protocol Dostribution (what they are doing) and IP Matrix (where they are doing it to/from). A final point to make is that the information you get all depends on the way RMON has been implemented in the hardware you are monitoring, however most RMON-enabled devices will do what you need
The administrator has disabled public write access.