My 2002 Jetta TDI.... lately is not starting and I

My 2002 Jetta TDI.... lately is not starting and I have to jump my vech to start. About 3 months ago my car my started acting kinda funny. It wouldn't start without a jump and I really believe it is the battery. However, I got the battery tested twice and both times they told me my battery was fine. Here's how the problem started every once in a while, once or twice every couple of weeks my car wouldn't start... it didn't have enough juice to turn the engine over. Now this problem is almost everyother day and I don't know what to do without spending a ton of money to have the dealership check everything. Has anyone had a simular experience? What was your solution?