Mahler Vertical

Hi guys, I bought the Mahler, obviously used and paid very little; the problem is that it doesn't hold its tuning. Is there a way to regulate it properly or do I have to sit there tuning it every time? What could it depend on? Speaking with a tuner, he told me that these Mahlers don't hold it. Also, I have a problem where one of the three strings vibrates too much, how do I solve it? Do you think it's the damper? Thanks so much