You are right to expect quite a bit more then 1% ... !!
I'm presuming you are using cables in good condition, and that you haven't set any bandwidth goverance on your modem (e.g. QoS stuff for VOIP)?
Are any of your cables run close to transformers, electrical motors or other devices which induces a large electro-magnetic flux?
I don't have network trouble-shooting experience to be able to suggest more then logical possibilities, but hope even those could trigger a resolution.