Car starts sometimes

Ok to start off I have a 1999 Saturn 3 door coup. My problem really started a couple of weeks ago. One day my car didnt start so I try to get a jump from my gf. No luck the car wont start. An hour later I jump it with my brothers car and it starts right up. Two days later im at work and I go to leave my car wont start again. 3 different cars tried to jump it but it was still dead. So I have it towed to an autoshop so they can take a look at it the next day. Well the next day they try to start the car it starts up fine again. It was there for almost 2 days and the car started up fine everytime so I took it to the shop for nothing and stuck with towing fees. Yesterday I try to start my car after leaving the store again it was dead didnt start. This time I didnt try to jump it but left it for 24 hours then this morning I go back to the store and it starts just fine. At the autoshop I took it to the mechanic says if the car starts fine while its there everything will show up fine on a diagnostic test. Anybody know what to do here or have a similar experience. I know if I have it towed next time it doesnt start after it gets to the shop it will start up fine again its very frustrating.