Starter not working all of the time? Let it sit for a while and it pops off. Nine times out of ten it is a bad solenoid. Bad solenoids tend to draw too many amps and over heat. Replacing it with a Nissan unit and your problem should be solved. Another solution is to get a gear reduction starter that is commonly used on Nissan forklifts. These are smaller and spin the motor a bit faster for quick starts. I do not have the part number handy but will look for it so I can post it here.
If your starter goes bad it can be a real pain to replace. Replacing can require you to remove the intake manifold and carbs at a minimum. Loosening the drivers side motor mount and jacking the engine up to give you more room. The best starter to use is the short nose 2 liter starter. It gives you a bit more maneuverability than the standard long nose 1600 starter. If you peer into the drivers side wheel well with the tire off you will notice a triangular brace running from the inner fender to the firewall. I drill out the spot welds and remove this piece. I then fabricated a piece of aluminum to fit back into the spot and bolt it in place. This allows you to remove the starter without removing all of the above items plus it looks better than leaving the piece out completely. Saves a lot of time and energy.