Hi Mathew,
I run multiple exit nodes in the US and have what could be the same problem. My first node 'apexio' has been running with 99.99% uptime for four months and the bandwidth usage is minimal and dropping. I'm using Linux with very fast hardware and ample resources.
I mentioned all the specifics on this list a couple weeks ago and no one had a solution.
A customer of mine also has the same issue.
Something is going on that's NOT related to hardware or network speed, but is a symptom of some issue in the Tor network.
Recently I turned up nineteen additional nodes on that server and they're averaging 60Mbps of overall throughput. CPU load is still 0.00.
-Jon