intermittent problems are tough. Another issue is that those really don't have a couple of known issues. This is going to be a 'pin it down when it happens' deal.
I would check grounds and connections, but it sounds like something is failing open occasionally. You can throw parts at it, or try testing WHEN it's doing it, but otherwise, you are just going to spend time staring at it when it's working like they did. Frustrating, but that's the nature of the beast.