Is comcast throttling my connection?

For the past week I have been experiencing what can only be classified as throttling.  I have the 12mg Business class HSI.  If I do a speed test I can get 20M.  If I connect to a server to make a large download I will get 20M for 15-30 sec then it gets throttled back to 7-8M.  If while I'm downloading at 7-8M I try and run a speed test I can't get more that 1M.  I should still have some bandwidth left if I'm only using 7-8M.  I have connected directly into the comcast router/modem to eliminate any of my equipment, ran the download on one machine while doing the speed test on a separate machine and it is still the same, tried hard wired and wireless.  It doesn't matter what time it is, even at 3am I was still experiencing the same issue  Comcast has sent out two techs and neither of them have seen this and left scratching their head.  Both techs ran through all the usual, resetting, unplugging, making sure all the connections were tight eliminating any of my equipment, etc. The second tech wanted to replace the modem/router but didn't have one on his truck.  I think the techs are chasing after a problem that does not exist on premise.  If it was part of comcast traffic management system it wouldn't be consistent.  Obviously I have spoke with telephone support and I am reassured that my connection is not being throttled, but they don't know why I am experiencing throttled speeds.  Please help-