Bandwidth drops 90% or more when downloading

Hoping someone might have experienced this weird issue before and knows of a potential solution.  I am currently subscribed Comcast Business service.  Normally with no activity (I.E. No downloading) I run at 35mbps down and 11.57 mbps up (http://stage.results.speedtest.comcast.net/result/385101324.png) .  As soon as a download over 3mbps is initiated my download and upload drastically drop, 6.42 mbps down and 3.56 mbps up (http://stage.results.speedtest.comcast.net/result/385106264.png) Once the download is completed speeds restore to normal.  I have been dealing with this issue for months and finally decided enough was enough and contacted Comcast in November.  Since then I have had my modem replaced, multiple techs from Comcast onsite, replaced my layer 3 switch, replaced all cat5 cables and no avail.  I know the issue is not in my residence but potentially with the line in the junction box, Comcast keeps promising to send out a line tech but no one ever shows or calls to update.  Would appreciate any insight and help on this issue, hate to get to the point of cancelling my service.