This complicates things somewhat - since hops 12 only shows light packet loss. We can't conclusively say that hop 11 is the culprit, in fact this even argues that the problem may not be hop 11. If your ISP is looking for an "out", then this data won't help your cause much. There are a number of things that could cause this type of picture, though - including problems at hop 11. Usually, though, it's a problem with a router that can only be seen by tracing the reverse direction, though. Another possibility is that there are frequent route changes coming that are disrupting the data collection to the final destination.

Do you have control of the far end of things? Can you reverse the trace and run it the other direction? That would probably add some clarity to this situation. If you have control of that site but don't normally work there (or have easy access to a remote computer), you may be able to install the remote agent there so you can run a trace from that side while sitting at another site (http://www.pingplotterpro.com/remote_trace.html).

- Pete