If you drive your car again before you fix it try this. When it dies or after you have driven it for some miles (some miles is between a bit and a lot) put your hand on the coil. If it is working OK you should be able to keep your hand there. If it is going out the coil will usually be too hot to hold. This was true with old style coils that ran point type ignition systems. I am not sure if it still holds true for the electronic type. I would imagine that a diagnostic shop could tell you in the coil is going out.