Given that all the plugs are showing some buildup in a short time your thinking that it might be the coil is sound thinking, thouogh cylinder #1 has some particular problem the other's don't. I'm assuming your mechanic checked the timing. I'm totally in the dark regarding an m103 engine, but if the timing has initial distributor advance, vacuum advance, and mechanical advance (spinning weights), it would be a good thing to know the entire timing advance curve from idle to all advance in. You mentioned checking the resistance across the distributor; also make a visual inspection to see if there is any corrosion in the spark plug wire end as well as the contact inside of the distributor that might be missed by just sticking your multi-meter sharp probes in. Pardon my ignorance of your engine, as perhaps none of this applies. Is your distributor vented? If you're in a wet or humid climate check for any moisture droplets within the distributor cap. If the combustion chamber is heavily fouled, hot carbon spots might be causing that cylinder to pre-ignite, thus fouling the plug. Since you seem to be having a problem, though lesser, on the other cylinders, I'd agree with replacing the coil and having the whole timing advance curve analyzed. Also, make sure the #1 plug wire is not sitting abnormally close to any hot engine parts; actually, make sure all the wires are kept as distant as possible from hot engine components. Heating a copper wire causes the resistance to increase.