That looks like a hardware issue - but obviously if restarting the software helps, it must be something else... unless the downtime from the restart is giving it time to cool?

Unfortunately, I never received an Avalon Nano sample to debug with - IIRC, the vendor only wanted to send a sample if I was going to personally buy a large volume myself.
I'd recommend customers express their interest in well-tested BFGMiner support, so that perhaps they will understand the importance of sending a developer unit.