I'm having the same problem, but only on one share that's running on the first controller. Our other two shares on the second controller work fine. The settings are all the same, the only difference i've noticed is that when copying the large file, the share that fails is actually copying twice as fast as the other two shares that work.
It seems to be a SMB problem with ONTAP 8. I tried to copy a big file from a NetApp filer with ONTAP 8 to my local Win 7 PC with windows explorer. The copy starts with 2 MB/s or less. Within a few minutes the copy slows down to < 1MB/s. Only reading is a problem, writing works fine. Also reading from a Windows Server 2003/2008 works fine. Seems to be a smb problem with ONTAP 8.
The same situation on a NetApp filer with an ONTAP 7.3.6 works fine.
Our workaround is:
Run Cmd with
netsh interface tcp set global autotuning=disabled