1985 Mercury Cougar Battery problem?

I have a 1985 mercury cougar, I bought it about a month ago, and its been running fine. All of a sudden one morning I go to start it and it wont start, it just makes this loud clicking sound. So I got a jump and it was fine, until I turned it off. After the jump if I turn it off for twenty minutes or so it will run again, but if I wait an hour I notice it will just start clicking again. I went to autozone and had a free testing and it said the battery was dead, so they charged it and it came out fine. So I put the battery back in and it ran fine for a day, then when I get up to go to work in the morning, I went to cut it on and it sounded like the car was having a hard time staying running. It ran for about 5 minutes while warming up, then the lights dim and radio cuts off, so I turn the car off and tried to start it back up and thats when it started clicking again. Is their something draining the battery or a possible shortage anywhere? Your help will be gratly appreciated.