Mac Pro network speed spontaneously slower

A few weeks ago, I copied files from my MacBook Pro to my Mac Pro over a gigabit network and reached a record 50MB/s. But upon my return from Africa last week, the network suddenly became sluggish; copying files over the same network is suddenly capped at around 800KB/s. The slow network seems only to show itself when copying to or from the Mac Pro. I've verified it by testing transfer speeds from the Mac Pro to multiple machines, and to a NAS box (also on the gigabit network). Copying files from the MacBook Pro to the same NAS box yields normal (fast) transfer speeds, so it seems that is must be a Mac Pro issue.

Does anyone have suggestions for how to diagnose the problem?