[MOTORHEAD]

Use process of elimination; Put the pass side plugs on drivers side, and vicea versa. If problem switches to drivers side, it's a bad plug. If it stays on pass side it's not plugs.
Replace the dist cap next, some electrical defects cannot be seen.
Replace ign wires on pass side.
Still no joy?
Pull pass side header and look for signs of leakage. A leak will leave a soot trail, or other gasket damage may be obvious.
still have problem, try a new coil.
Belay all the above, you have a backfire, not a miss.
Could be induction or exhaust leak.
Are you sure it's not a miss ?