Can you rebuild a Sram gear hub? 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.
I'm very skeptical that an internal fault in the hub is causing the external mounting of it to loosen up... unless you are experiencing frequent total rear wheel lockup and skidding, maybe. I have a SRAM P5 hub that I built into an old mountain bike myself. Originally I ignored SRAM's torque instructions for the rear axle nuts and just tightened them by judgement. I have rebuilt car engines this way in the past, with no problems. But after having my rear wheel loosen up, I got out the SRAM specs and a torque wrench. Let me tell you, they specify cranking those nuts A LOT tighter than I would have thought, or dared to do without the torque wrench.

I'm also skeptical that nobody in Denmark fixes internal-gear hubs. Never been to Denmark, but thought bikes with such hubs were very popular out that way.