1992 Lexus LS 400 Starting Problem

I have had the car about 2 years, just recently it has a starting problem. Every so often I go to start it, and it acts like it has a dead battery. It can go one day up to a week with out having to be jumped to start. I've had it start after sitting a weekend, then i drive it, stop at a store and when I turn the key all I get is a click. I have also had just a click, then I turn the key to the off position, try it again and it will start. I have had the battery tested, AutoZone said it's fine. When they tested the battery it wouldn't start, they put ther portable jump box on it, still wouldn't start, had friend come by and jump it- it started right up. The lights never dim, like an alternator problem, don't think it has any type of electrical draw. Due to the fact it starts after sitting, and its so speratic, I have to carry Jumper cables every where I go. I NEED HELP, its getting to be winter time, and i need answers.