Too large a gap can cause the HT system to operate at a higher voltage, leading to problems over time - in some cases burning out the rotor which in turn leads to the cap burning out. So that is the first thing I would check. Then ignition timing and the engine (cambelt) timing. I also always,whenever possible, make sure the rotor arm and dizzy cap are made by the same manufacturer. It reduces the likelihood of the rotor arm being out of phase with cap.