Intermittent engine cranking issue?

So, about 2 weeks ago my car started to have problems starting. It sounded just like a battery dying. (The drain battery light didn't come, something I just noticed) I went to get it replaced and the car started up great! A few days later, the car wouldn't start again. It would crank, just like the first time. I had my car jumped and it was fine. Unfortunately, just like before, it did the same thing after a few days. I was able to start it up after a few tries without getting a jump. I took it to the shop, and they said nothing was wrong with it. Well, a few days later and. It wouldn't start. This time, even after a jump, it wouldn't start! I thought I had to get it towed. I left it in the parking lot and came back for it the next day. It started up with no problem! I went back to the shop, and they fixed 2 things. They replaced my spark plugs, and they replaced a sensor. (I wish I had more details on the sensor, I don't have the receipt from the mechanics at the moment), so I went back to pick up my car and it didn't start again! My car has been with the mechanics for about 2 weeks now (off and on) and the car starts for them every time. They said after testing the car that nothing came up for them. I guess the fuel pump is fine. I just don't know what to do. I'm afraid to drive my car because I don't want to get stuck, but the mechanics can't find anything wrong! Thanks for the help in advance!