2003 SAAB 9-3 repair Question 2003 SAAB 9-3 Engine Shuts Off

My car recently hsut off while driving. It is like someone turns of the key but everything continues to work fine. The first time it happened, when I tried to restart, the engine spun but would not start. I had it towed and the mechanic told me it was a loose battery cable and the he tightened it and reset the computer and it was working fine. Not even 2 days later, I was driving and it just stopped again. This time when I tried to restart, nothing would happen. Everything else worked fine. So, I disconnected battery, jumped in and it started right up. This has happened a few times now and does seem to be getting worse, or more frequent. The other day to work, it stopped, I disconnected battery and it started but stalled right away, so I restarted, and it stalled, So I disconnected again and it worked fine. Very confusing to me and I would hate to be charged another 100 for a mechanic to tighten my battery.