The pattern you're seeing looks a lot like a piece of hardware that doesn't like responding to multiple ICMP requests with TTL=0. We have a router here that does this - and if you trace to multiple targets one of the hops toggles between packet loss in the different targets. It always responds right on one target, but the other ones time out.

The fact that it doesn't affect downstream hops is an indication that you shouldn't put much weight on it. More details here:

http://www.nessoft.com/kb/24