1999 Mazda Protege Some type of electrical problem

1999 Protege is only driven a few times a week so it sits in the driveway most of the time. Last December I drove it a short distance to a store, went in the store, came out a short time later and it acted as if the battery was missing. No power to anything. Would not take a jump. (A few months earlier I had replaced alternator and battery.) Had it towed home, let it set in driveway untouched for a week. Got in one day and it started. I did nothing to it. Since then I have replaced the belts, radiator, andjust had A/c condensor hose cleared of a clog that was causing water to leak into the front passenger side. Yesterday my son drove to a nearby restaurant, had dinner, got in started with no problem and drove to another store. Went in the store, came out started the car then turned it off because he was looking for CD. Found the CD, then tried to start the car and it acted just like it did in December, He called me I went over and looked at it, and again the car was completely lifeless. No juice at all. About a half hour later we tried to start it again and while it did not start or even begin to turn over it did show some sign of life. The radio light flashed on and normal dash lights popped up for a second and then dead again. Before I take it to the shop again, I would like to have some idea of what the issue could be. Any help would be appreciated.