My YJ is cutting off for some reason!

I bought a 1993 Jeep YJ Wrangler last week and finally joined this awesome family! over the last few days, it has not wanted to start. Yesterday and today i drove it from work to home and turned it off but needed to restart it to go run some errands in town. When i go to restart it, my dash lights up and then it clicks and everything goes out. When i turn the key again, nothing happens. Yesterday, i jumped it and took it to Orileys auto parts and they tested my Optimum battery, alternator, and starter, which is brand new. all checked out. i removed my winch cable from the battery terminal and tightened it back up. My jeep cut right on. i took it home and then to work today. i left work and came home pulled in the garage cut it off, ran up stairs to change and came back down. no start. will some one please give me some incite on whats going on!