No simple answers but you might try the following to help find out what's going on
1. Install
Hashtabs
Now you can easily see the CRC values (CRC32, MD5 and SHA-1) for any file on your machine. Use this to see if the exact same file download gets the exact same CRC for each download. Just select the file, rt click
Properties, click
File Hashes tab
2. Check file sizes
Each time you download a file with errors, is it the same size? Or does its size vary from time-to-time?
3. Usually more a problem with very large downloads, but try changing your adapter settings to see if helps
> Open Device Manager. Click
Start->Run, enter:
devmgmt.msc
> Look under the Network Adapters category for your network adapter device
> Rt click
Properties. Is there a Power Management tab?
> If yes, click it. Then uncheck the box so
not to "Allow the computer to turn off the device to save power"