We talk about this topic a bit here:

http://www.nessoft.com/kb/24
and here:
http://www.nessoft.com/kb/5

The most likely problem is that hop 5 is down-prioritizing the creation of ICMP TTL Expired packets, but it's returning packets for other routers just fine. That's not abnormal. When you directly ping that router, it's creating a different type of reply packet (ICMP Echo Reply), which probably has different routing rules.