DLM systmes play around with the ADSL metrics i.e. connection speed, target noise margin and interleaving. Not dynamically throttling the IP throughput every few seconds.

More likely your throughputs are really always around 14.1 Mbps, but looking at the tbbmeter there was a small gap before the 17 Meg peak, i.e. might just be a factor of how your AV suite is operating on trhe PC

Thanks Andrew but if it is AV I have tested on 3 machines with two different AVs, two XP machines with MS AV and Nod32 and a Win 8 machine with MS AV. Results are identical. The small gap at the beginning is Firefox switching from single thread download to mutiple threads to max out the connection. These results are repeatable. Single thread downloads are rubbish.

Just to round this thread off with some sort of conclusion. Vivaciti and I have agreed this appears to be exchange congestion, but there are no plans for the wholesaler to upgrade capacity at the moment especially as there is little impact on my LLU line. Will have to keep an eye on it and hope it doesnt get any worse or I will have to shift to BT 21CN and their DLM.