Ditto etemplet's comment about small ground wire. I had similar issue with my 96 GTX and it was a matter of patience tracking down the problems. In my case, the cable from the battery to the starter had internal corrosion and needed replacement as well as the small ground wire (you can't always tell by looking, use a multi-meter). But the final issue was a loose connector in the front mpem box that fed into the relay holder. When it's loose or corroded, any electrical spike can't "cross the bridge" and therefore significantly reduces a required amperage spec required to complete the circuit. The troubleshooting guides in the manual address this. Be patient and put on your detective hat.