I just discovered this Comcast user forum. It describes the same problem we are seeing.

http://forums.comcast.com/t5/Connectivity-and-Modem-Help/Peering-problem-with-1and1-EU/td-p/1420389

Responding message says:

It could be a peering / interconnect capacity / congestion issue between them. Have you reported this to both CC and Level 3.

Any idea what this means, David?