@xuraax, that's why I suggested to do all this "cable testing".
Read rare cases of simply faulty isolation on sparkplug cables and other parts of your harness.
Had an ignition cable with a tiny hole in it, through which it fired towards frame when warmed up, causing similar symptoms like yours.
Changed coil, cable and sparkplug cap, spook gone.
Have to say, was on a non EFI motor.