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This has been an on-going problem ever since I installed a Delco Voyager ignition system on my starboard engine last fall. I check the wires before I leave the dock and all are properly seated on the spark plugs. After running about 10 or 20 minutes, I hear the starboard engine loose synchronization, dropping about 10 rpm. I check the spark plug wires and find that one, usually the #7 or #8, has slipped off of its plug. The wires seem to be securely attached to the plugs, requiring a good bit of effort to pull them off.

Any ideas what might be going on here?

Try carefully snugging the metal fitting with a pair of pliers

Also try working the rubber boot down as far as you can.

SP wires on your typical plug should snap on as the tip of the plug goes into boot metal female connector.
If they do not the boot is bottoming out on the plug shoulder or the connector needs a slight crimp.

If they are straight boots, pull the boots back and see if the connector will snap on and push the boot up onto plug.
Dielectric grease works well as a lube

If they are 45 or 90 boots, pull wire back out of boot and try connector on plug tip.

The plug wires on my C115 Yamaha are very "lightly" attached to the plugs. It's more of a design issue than anything else, so I snug the boots tight to the plugs with wire -tys for added insurance.

I have found some need to be “burped” as they can trap air and it pops them off.

A small wire tie is an old racing trick.

I replaced the wires which came from my EST ignition systems and the problem went away. The new wires were a couple inches longer than the old ones, so it is possible they were under some tension and were being pulled loose.