Contact planet bike customer service. I had the Beamer 5 (which I think you're referring to) start to act up in a similar fashion.
Mine turned out to be a faulty seal on the switch. It would sometimes go into what I called "induce epileptic seizure mode" (especially after seeing rain or snow) and it would be totally unresponsive to the switch - couldn't turn it off or into solid mode and so would have no choice but to induce epileptic seizures wherever my route took me. Kind of like Johnny Appleseed, but not quite so nice.
They sent me a new one. I put a bit of electrical tape around the seam now and haven't had that problem since.