It could be anything. I would start with a thermostat, or possibly an electrical issue. I hope it is not the water pump but like I said, it could be anything. The only thing the A/C would have to do with it is that the fans come on with the A/C but you said they were working on thier own. Also there is a small chance of your cooling passages building up scales and rust and shit on the inside of them restricting the flow, I doubt that would be it but I have seen it before. I would replace the thermostat, do a cooling system flush, and see if it still does it. Just for shits and giggles, make sure there is no debris (leaves, paper ect..) on your radiator restricting air flow. Good luck, I hope it is something simple. And good choice on a DD, I love old Maximas. My first car was an 87 Max, and I had a 97 GLE at one time. Pre 03 Nissans are BULLITPROOF.