I bought a used bike with Sram 7 gear hub and after a few weeks, the chain started falling of all the time. I took it to a bike shop and they told me that the problem is in the gear hub, that it's not working as it should and because of that, the rear wheel keeps moving out of it's place, out of the dropouts. They also told me that probably no shop here in Denmark would be willing to do the rebuild of the hub. 

Is this true? The cost of a new hub is more than 50% of the price I bought the bike for, so it doesn't seem reasonable to invest any more money in it.