First of all, thanks for running a relay.
Based on my experience, what usually happens is that the provider of your VPS observed during a period of time you used more than N mbps constantly and all the time, so they capped your VPS at some KB/s limit. There are performance monitoring scripts that could do this automatically. A virtual private server shares the network card of the host with the other VPSes on that host, so almost all providers do not allow you to use it all by yourself all the time for long periods. You can open a ticket upstream and they will confirm if this is the case or not.
Nothing you can do about this unfortunately, most providers do this, even the ones they say they don't do it :) Only thing you can do is get a dedicated server with guaranteed bandwidth, or try to convince them to at least lift your the limitation for your VPS to 1mbps.
In this case, this is not going on as we *are* the provider. I'm a sysadmin on the network and I'm one of the guys that would be in charge of limiting any machines which violated any rules. :-)
Trey Nolen