Comcast throttling?

Similar to another thread on the list today, I'm troubleshooting a problem for a customer on Comcast business fiber. Downloading a file from one of our web servers is very slow (~15KByte/sec). mtr looks clean in both directions. I added an IP address on the same server from a different class C on our network, and downloads form this new IP are fast (2MByte/sec). Tracerouting from server to client is the same using both source IPs. But, one IP consistently has the very slow speeds that the other does not. Changing our outbound path between different upstreams does not make a difference. It certainly feels like Comcast is throttling one of our IP ranges. Could someone at Comcast please contact me off-list for details? Thanks, Mark