I'm assuming this is an HEI style replacement distributor? Are you using the DUI coil? What engine? GM can run hotter as the air circulation at the rear of the engine is not as good as the Fords up front.

Heat is usually the culprit. The DUI distributor does not recommend a ballast resistor, rather a direct battery lead - which is what you have. Did you put a blob of dielectric grease under the module(s)? Any brand will work and a small tube typically comes with the module. It's critical to use the grease and make sure the module seats tight to the distributor to transfer the heat away from the module.