Hey Chris,
I've seen errors like this before on both sides of a relay server and it pointed to, as Reg Domaratzki says, something in between your SMP server and your relay server interfering with your traffic.
I would start with a detailed network diagram - look for proxy servers, firewalls with enhanced security or any other piece of "intelligent" hardware that could be analysing and reacting to your traffic.
I've had so called "intelligent" firewalls before that decided that the persistent connection outbound from SMP was suspicious and therefore periodically terminated the connection. Or I have also seen transparant proxies that cut off connections with chunks of a size greater than 32767 bytes (that number stuck with me as a result!)
Specifically look at transparent network devices that you may not be aware of as those are often the devices that cause the most trouble 
Let us know how you get on anyway and hope you sort it soon,
Brenton.