N6458C - EAX in a former life when it had blisters.
Seems she is still grounded, possibly due dead-cut/backfiring L engine, L mag.
Any thoughts?
A mate of mine who reckons he knows about these things, suggests that if the mag. was replaced with an old used one, he’d be replacing it again with an overhauled one. Also, would be looking for an intermittent short in the ground wire which seems to come in at certain power/rpm range due associated increase in engine vibration. Dirty?
Try disconnecting ground wire and running with live mag. (but if does not fix, mag cannot be switched off)