I know you mentioned you changed the plug and the wire, so I assume that was with known good units.
As someone mentioned though, maybe simply swap #3 plug & wire with #5 plug and wire, and see if the problem follows. If it stays, you can rule out electrical. Then you know it's mechanical.
Just more process of elimination as you go. That's the only suggestion I can see at the moment, and what I would try next.
Hope you find the issue easily!
__________________
Eric Mitten
'74 Bonneville 4dr Sedan (455/TH400/2.93 open)
'72 LeMans GT (455/M-13/3.23 [8.5"] posi)
'71 GTO Hardtop (400/TH400/3.07 12 bolt posi)
‘71 GTO Convertible (455HO/TH400/3.23 posi)
'67 GTO Coupe (455/ST-10/2.93 posi)
Deuteronomy 8:3
