1989 Nissan Truck Not Starting/then starts

It started around a year ago. My truck just didn't start anymore. I got a mechanic to pop by and look at it, he said it is my Ignitition switch was loose, and to hold it down, it worked, for 2 months. Now I still have to hold down the switch, but 75% of the time, it will not start at all. I changed the starter on it and it worked fine for a week then back to nothing. It will just click most of the time or no sound at all. If I turn the ignition on off 100 times It might start. The starter is brand new, so I'm thinking it is a electrical problem, but where? I am a student and cannot afford a mechanic at this time, so any help on where to start looking would be greatful. Thank you