Perhaps it's a flaky ignition module...They do become intermittant sometimes.....

Also, I've seen similar symptoms when the ground lead is loose from the engine.....

Or, it may not be ignition at all!

I've had this kind of problem myself when I used a supposed gasoline stopper in a fuel tank (I think it was Sullivan that had marketed a lot of stoppers with the wrong color a couple of years ago, and later found out it was for glow!).

It started to disentigrate, and contaminated the fuel. I went through several flights with the symptoms you describe until she finally died (sucked all the goop into the carb)......

Just a thought.....