I found the problem (although another has been created).

It wasn't the filter or the line or the vent. It was the little clip thing that you click onto the engine at the end of the gas line. I just bought some new ones and they fit, although I did realise that they didn't "click" into place like the older ones (I have new one attached to internal tank line and old one attached to container line). So what would eventually happen is it would work it's way off the gas end on the engine and stop flowing gas into the engine, thus the periodic shut downs.

So I was happy to discover that.... but...