Intermentet no start

I have a 1995 monte carlo, 100,000 miles, engine runs great when it starts. My car starts good for maybe 2 weeks then it just wont start, makes a clicking sound, does this for a few minutes or sometimes several hours, then it will start. Has been doing this now for a month. The lights will sometimes come on bright when it does this and sometimes they wont come on at all. I have had the alternator and battery replaced, have cleaned battery cables from battery to starter. I had a mechanic run a diagnostic on it and he says all is ok but nuetral saftey switch. He tells me since the car will not start at all in anything but park and nuetral that the nuetral safety switch isnt my problem. He suggested I relace the key to my ignition, ( it has a computer chip), the chip is loose and he thinks this may be the problem. I will replace that today. Can a bad key cause this? If this doesnt help, do you think the problem could be my nuetral saftey switch?