Gas, or spark, well i have cleaned many a weedeater and chainsaw plugs. Replaced them in the engine and never missed a lick, I have had to respace the coils on normal ignitions to get a hot spark. A member at our club had a simular problem, found that his battery pack had a broken interal connection and I have had battery packs test good under load but after short use the plane was doing strange things. I discovered one of my cells had gone bad. I would get a new ignition battery to make sure, yours might just be lossing a cell or have a broken connection under the covering.

Will