Crusader 454 Vibration

Hi, all. New to the forum, and looking for advice. Have a 1988 Mainship Mediterranean with twin Crusader 454's.
Recently the starboard engine (reverse rotation) developed a hard rhythmic vibration related to engine RPM's. There's no noise associated with the vibe, and no evidence of coolant in the oil. In troubleshooting it yesterday, with RPM near idle, I pulled/replaced one spark plug wire at a time from the distributor (Mallory aftermarket YLM63), and when I removed the #3 wire the vibration went away and the RPM increased about 200 RPM's.
I assumed that the #3 cylinder was firing early, possibly because of a shorted distributor cap or a "jump" from adjacent plug wire. I separated the wires making sure they were connected to the correct plugs; removed and cleaned the distributor cap, and replaced the #3 plug, which was quite heavy with carbon but properly gapped. I've ordered a new cap and tomorrow I'll do a compression check. In the meantime, if any of you have any advice on this, I'd appreciate hearing from you. Many thanks.

Step #1 would be to confirm the firing order is correct:
LH 1-8-4-3-6-5-7-2 RH 1-2-7-5-6-3-4-8
Have seen your presentation before on big block car engines that were mis-wired.
Good Luck,
FL Panhandle

Thanks. Good news (and bad) is that I've checked and re-checked the firing order (It's the 1-2-7...... for reverse rotation) and the order is good. I'll let you know what happens next.
Take care.

Ignition wires would be suspect, especially if original. New cap/rotor? every other year.

diver_dave said:

Ignition wires would be suspect, especially if original. New cap/rotor? every other year.

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Thanks, Diver Dave. Looks like I'm headed down that path now. Parts should be here in the next few days. I'll let you know.