"a" wrote in message
...
Next time you're at altitude and in cruise mode, switch from 'both' to
a single bank of spark plugs. It will remind you what happens should
you have one side decide to go in/op when in flight. I had the engine
(IO 360) go rough a couple of weeks ago, but rough was a lot better
than the very smooth and silent effects the left mag only gave me.

Sounds like at least two problems--unless the fault was in the switch.

Peter
Just a thought