If you can tell when the spike is occurring in flight, do a mag check. I have had a similar problem and found that while the mag (ignition) check passes on the ground, it may fail miserably in flight. The engine should just run a bit weaker in either mag position, but still smooth. If it stumbles on one mag, the mag (or electronic ignition) or a plug wire may be misbehaving. An ignition check in flight is a good thing to do periodically anyway, even though it feels a bit intimidating.