One month ago, I would have said Popular, but since I don't work there anymore, everything is crumbling down... just kidding. Popular does a really good job.
But as Dubweiser and MrSpace said, you might want to consider going to an independent shop. Independent shops are more motivated to find gremlins because they're 90% sure they'll be the ones to fix them in the end (prices are cheap compared to the dealer's, so you'll comeback for sure).
Is the car still under warranty?