This sounds like a problem I had with my network when I was still using Dopus4.16. In fact, I'm quite sure that it's Dopus4.16's fault. After I switched to Dopus5.82 the problem seemed to have just vanished. Didn't like Dopus5.82 as a Workbench replacement, so I set it up to work like Dopus4.16.
However, I only had this problem when copying large files, so trying DSopus5.82 may not solve the issue.
