I suggest you buy a NGK LZTR5AIX-13 spark plug ( Stock number 2314)
and swap it into number 5 cylinder.
Inspect the OEM Champion RE14PLP5 spark plug carefully that came out of #5 cylinder.
Use a magnifying glass to see if both tiny discs are still welded in place. A bigger gap than spec is a clue that one or both discs are gone.
I use a trusty old hand cranked Biddle Megger Tester set on 1000 volts to test the internal resistor of the Champion "RE" plugs which have an unusual patented semiconductor element inside.
https://www.ebay.com/itm/Biddle-Major-Megger-Tester-21159-W-Case/282747408077?epid=686950419&hash=item41d50f0ecd:g:wQ4AAOSwaEhZI9tT
But that might be overkill.
Some experts have doubts about Champion's choice of internal resistor
http://www.vansairforce.com/community/showthread.php?t=90844While you have the #5 ignition wire off,
check resistance from wire end terminal to terminal and compute ohms per foot.
Wrap a strip of aluminum foil around the outer insulation of the ignition wire and measure megaohms of resistance between aluminum foil to inner carbon fiber conductor to see if that wire has a leak to ground.
Put wire and plug back.
Fire up engine.
Test drive to see if roughness gone.
Use stethoscope or long screwdriver to hold against #5 fuel injector and listen to see if its clicking noises are similar to other injectors.
Might not be a bad idea to look at the ignition wires in a very dark place to see if you can spot any light from corona discharge: that blue green glow.
