1997 Plymoth Grand Voyager

The past few weeks, occaissionally, my car acts as though the battery is dead. When it first happened I replaced the battery. It does fine for a few weeks and than it will do it again. Each time I have someone give me a jump and eventually it will start. This past week I took to battery back to where I had bought it, they checked it out and said it was fully charged. I took it back, put it in the car and it started right up. One mechanic did a diagnostic check and told me everything checked out fine. (Alternator, starter, fuel pump)
Anyway I just went out to start it and once again, it wont do anything. It did click once, and nothing else.