Ignition miss at altitude/high power

[datorres88]

I have been experiencing an intermittent ignition miss on my #3 cylinder while in cruise at altitude. I should mention that this is occurring on my EIS (electronic ignition), not on my mag, but I think in this case the principles would be the same. I think I may have isolated it to a bad ignition wire. I have already changed the plug, so that was ruled out. When a perform a run-up there is no indication of any problem. I have performed a mag-check at full rich mixture as well as doing a mag-check after leaning to a very lean mixture (which should be checking the ignition system under the most unfavorable mixture conditions) and there is still no indication of a problem on the ground.

My question is, what would be the most common cause of an intermittent ignition miss at altitude? I am thinking that there is really only 4 possible causes:

1) Bad plug (ruled out).
2) Bad ignition wire (Thinking this is the most likely)
3) Bad coil (Doesn't seem likely since the EIS uses the same coil for #3 and #4 and there is no issue with #4, although there could be a connection problem to the #3 tower, yet still seems unlikely.)
4) Wire harness from control unit to the coil pack (also seems unlikely since other cylinders fire.)

btw, I should add that I have a JPI 830 and identified #3 as the problem by doing an in-flight mag check and looking at EGT.

So, what does everyone think? Is my thinking correct that I should suspect the ignition wires?

 

[EchoDelta]

 

[datorres88]

 

[philiplane]

 

[Cgach]

 

[datorres88]