Well I think the problem is your wireless connection but to help you better you need to provide what specific equipment is connected. The NAS is not wireless so what is it connected to. The computer sending files, how is this connected to wireless as well. If you're using two wireless routers, are they bridged? If they are routers, are they using third party firmware?
Either way, I think you will find the problem is in the wireless connection. Look for signal strengths and see if they tend to drop periodically.
Your feedback will help us assist you. I suspect you will end up posting this problem on a forum for your wireless devices.
-Joe