2007 Dodge Caliber car won't stat unless I get a jump

my car wouldn't start, and I had to call a toll car company to give me a jump, and my car started right up. now when got off from work it wouldn't start up, and again i have to have somone to jump my car. Now I took it to the shop and they told me that it was not my starter or battery everything was find and the told me they can't find nothing until the car start acting up again. what do you think the problem is.