thanks for your quick response. In the meantime I am in contact with the ISP who "produces" the packet loss. At least I hope its them. Its pretty hard to find out who's the bad guy in the chain. I did use mtr on my mac to trace + ping all hosts on the route but that didn't turn out any useful information to me.
Am I right, that from Wireshark capturing there is no way to reconstruct on what route a packet got lost?