1994 Isuzu Amigo Starting

Once every couple of months, my Amigo just won't start. I turn the key and nothing, no clicks, no sounds, nothing. Lights and signals and radio all work. I have tried to use jumper cables, still nothing. I usually roll start it being a manual. Then the next time I try to start it it is just fine, again for weeks or months. Any ideas what I should be looking at? I replace the battery a year ago and the starter 4 years ago.