"Yep - have the stator tested
"Yep - have the stator tested at a shop (needs a DVA tester, not something most guys have laying around in the garage - normal multi-meter won't work).
The 115's have what's sometimes referred to as a 2+2 ignition which only fires 2 cylinders at idle but all 4 as the rpms increase.
The low speed stator bobbins are used to fire 2 cylinders at idle (upto about 1800 rpms or so) and then the high speed bobbins kick in to fire all 4.
If it's dropping 2 cylinders above 1800 rpms then it is most likely the high speed stator windings that are shot (but the entire stator is one unit and must be replaced).
If it's more of a "phantom" thing with one or two cylinders firing "when they feel like it", then it is more likely a trigger problem (which is very difficult to troubleshoot with a DVA because it will always work when you are checking it).....
The stator test, however, will either confirm or quickly rule it out as an issue...."