i've gotten good results testing coils with ohm meters only to have them fail under load. they rarely quit, but they do get weak and still throw spark. that's why an ignition scope is needed to check the coil while running. failed alot of coils of that particular design that way. another sign of old age is the white or gray area that develops around the bracket. that's a sign that the coil is arcing to the outside-least path of resistance. good luck, chuck.