do you have 12 volts on the + side of the coil with the switch on? If not
you have a voltage supply problem. Could be some corrosion or a loose
connection. You need to trace the voltage when it's not working or you will
be spending money on parts that you are guessing are bad and could still
have the problem next time out. I'd say take a meter or a test light and
find out where the voltage stops next time. You will most likely be able to
fix it or at least jump it till you get the correct parts to fix.