Stu,
First off, I would connect or plug any loose vacuum hoses. While you're at it, check the lines for cracks or loose connections.
When you had the distributor cap and rotor off did you check to make sure your centrifugal weights were working properly, i.e. not rusty or sticking. They should move easily and return smoothly. While there, if you can, apply a vacuum to the advance cannister to make sure it's still working.
Also check the carb air filter and ensure the choke is opening when warm. The plug fouling may be due to a rich condition because of restricted air intake. You might also consider using some spray carb cleaner, both inside and out.
First off, I would connect or plug any loose vacuum hoses. While you're at it, check the lines for cracks or loose connections.
When you had the distributor cap and rotor off did you check to make sure your centrifugal weights were working properly, i.e. not rusty or sticking. They should move easily and return smoothly. While there, if you can, apply a vacuum to the advance cannister to make sure it's still working.
Also check the carb air filter and ensure the choke is opening when warm. The plug fouling may be due to a rich condition because of restricted air intake. You might also consider using some spray carb cleaner, both inside and out.