Make sure you define an explicit firewall or port forwarding rule to allow the ports which your sharing service uses. I have seen this kind of thing cause very annoying drive disconnects when you need it the least.
Another thing which can cause an annoyance like this is the router's ability to arbitrate a large number of wireless connections which have specific ports assigned to them. As the ports open and close you get to a point where the device needs to flush something to keep operating. invariably it will get something wrong as it rolls the dice with the oldest connection the router is aware of.
Also transmission errors can also cause this sort of problem. Typically the protocols have a specific period of time before they completely time out. Once the connection to your "NAS" completely times out, the connection to the device is dropped. You can sometimes tweak the TCP settings to aleviate this problem.
Wireless interference will also cause the same kind of annoyance if you have another access point banging away transmitting on the same frequency you are using.
Hope this helps you...