(2) possibilities I know of not Failing related.
1) Battery low enough to cycle unit off - I've seen that a few times
2) upper contact needs a good cleaning. - My 70w is fine but I had to take the side off one of my 30w's a few days ago. I lifted the upper contact out of its mounting and cleaned it with an emery board. The mod was shutting of intermittently and when tightening the battery cap. Works like new again.
Nice to know, just in case!