2001 Buick Regal My car just turns off

Hello, I just brought a Buick regal 01 about 2 weeks ago and the very next day while I was at a light, it completely shut off on me. It came back on within two minutes and seem to work fine the rest of the day. The following day it shut off on me again while driving. It totally just went off. It took me about five minutes and it came back on smoothly. Now I know the gas gauge isn't working properly but I had a full tank of gas. Later on that day I was in my car (parked) listening to the radio when I heard this clicking rewinding noise. I popped the hood and it seem to be cooming from the engine. I got back in the car and tried to turn it on and this time it did nothing. It made no noise in my attempt, no engine sound or anything, just dead. Well I came back the next day and it came on fine and ran smoothly. A mechanic friend of mine said I needed a tune up. I got a dianostic at this specialty shop also and they couldnt fine anything. Nothing was wrong with the engine or trans or anything like that. They told me it has to stall in order for them to find the problem, but it didn't stall the whole week they had it. They suggested I might need a crank system or something like that but wasn't positve that was the problem. Can someone please help me because I want to get this fixed.