Intermittent starting issue

No the Battery is less that a year old but that was my first thought so I took it back to AutoZone and they checked my battery and alternator and starter and they all tested okay.

I do not drive much and sometimes a week will go by without starting my car. It started out that I would try to start and it would just stutter click and then it would turn over and start. But it progressively got worse, to the point that I am scared to go anywhere for fear of getting stranded. So I bought a portable jump starter and carry in the car all the time. Have used it once. But I am not sure if it was the jump starter or the car just decided to start. If it turns over it starts.

I have a feeling that it is a sensor problem as I have also been driving down the highway and the motor stops running and then restarts on its own. This has happened two or three times. If I take my car back to the shop it will probably cost me $500.00 for a problem that I could fix for the cost of a small part or a fuse. Could it be a starter solenoid?