Stranded in the heat. And in the cold

My 2002 Ford Explorer will sometimes not start during the summer after driving it and stopping for a while, during the heat of the day. It will start later that evening when the temp cools down. In the Winter, the car has a different problem where the Security light flashes and the car will not turn over. I have to disconnect the battery for a while to enable it to start. I haven't determine how to create either problem at will, so it's hard to fix. What could be going on here, and is there one or two problems?