[Artmasterx]

I had a Crown Vic that did a similar thing after the intake manifold cracked and somehow let coolant near the plugs and coil packs.

If the engine has coil packs, then i would suggest simply replacing the one on cylinder 5. They are probably less than $50 at autozone and should be a relatively easy swap if it is anything like my Crown Vic was.

I am not sure if AZ would let you return a coilpack if it turns out not to help, but they might.