Modem disconnecting with large downloads

I've got Internet Essentials and have a Motorola SB5120 modem (which is probably a bit long in the tooth).  Everything works fine, except when there is a large amount of network traffic because of a large download, such as downloading Steam games or (at the moment) updating the maps on a GPS unit.  When downloading a large file, the connection drops, with lights dropping on the modem.  Modem will reconnect and work for a few minutes and then drop again. Only during large downloads--streaming is fine, gaming is fine, etc.  Suggestions welcome!