I would think it is time to replace it, although I am assuming when you say in motion you mean when you hit little bumps and things? It would have to mean the thing is not as immune to vibrations as it should be when working properly.
I am not an expert by any means but I do know that for the most part, these days it does not pay to repair things. I remember about 7 years ago I attemped to have a VCR repaired. It was but it was never the same, the same things or different things just ended up getting worse.
So, unless it is very obvious and very cheap to fix or replace a part, I would say it's time to chuck it.
I don't own any Alpine stuff but they make good quality items so you may want to call or email them with the same question? Heck it may be a cheap fix, worth a phone call or quick email....