Mac book air not turning on

I have had this problem for about a month. 5 trips to the store and nothing has changed. Replace logic board, track pad, wiped it clean and reinstalled entire OS. Still shut down randomly and wouldn't start up for hours! And, then only after about 20 SMC resets and a few other keyboard tricks with support on the phone. Then by accident, I left it open 4 days ago. Next time I needed it it was still going and has not shut down since! Back to the store for the 6th time to see what is causing it. I can't carry around an open laptop all day, but at least it is working. Hope this helps someone. Apple was really excited when I called them about it and they asked me to take it back again.