I have an issue with my Audi that my mechanic cannot figure out. Hopefully someone has some good ideas!

(Im not a mechanic...please ignore the non-mechanic terms!)
When driving, it will sputter and then turn off. I just got a new battery, timing belt, water pump and crank sensor. Not cheap, but it still happens. My mechanic says he has exhausted his resources and doesnt know why it happens. He hooked up the diagnostic reader and the only thing that comes up is a O2 sensor that needs to be changed, but that shouldnt be making it kill. When it does kill, I hook up my battery charger and that usually works to get it going. It doesn't happen everyday, but probably once or twice a week. Its frustrating to be driving down the road at 60 mphs and have it kill, try to get to the side of the road and deal with other drivers...

Any ideas?! Thanks for the help!!