Car dealers are pretty clueless about car audio systems. It’s not what mechanics were taught how to work on. It’s a sideline, at best, for them. It sounds to me like you have some intermittent problem in your system. I agree with Roger124, you should make sure you’ve reported this and have documentation that the dealer looked at it during the warranty period. Then, when it fails solid later on you should be covered.
Also, stock stereo systems in cars are notoriously low quality. You might just want to check out replacements from Crutchfield or some local car audio shop. You’d be surprised at how cheap it is to improve on quality by going aftermarket. I’m betting your head unit has problems, by the way, but I’m just guessing.