You really need a download manager for files over 250MB. Your ISP [or anyone else on the route from your data to you] is likely to start caching data... not always correctly.
Your browser may be to blame - it would be simple enough to test another - but a download manager will stand you in good stead for many things. Lots of them have plugins to intercept browser downloads, so you don't forget & get a broken file.
This is the advice section from an online service I use...
Personally, I've never tried their recommendation of ReGet - I'm on Mac & ReGet is Win-only software. As for PKZip - I've never had a broken zip file since I started using download managers
Downloading large files over HTTP on the Internet can be tricky. To
avoid the frustration of broken or corrupt downloads, observe the
following suggestions when downloading from the Zip Manager:
Never download any
zip file or zip segment that is greater than 255 Megabytes (MB) in
size. If your zip file is greater than 255 MB, use a download manager
to split the download into multiple parts.
This is a limitation of broken, or misconfigured, web cache servers
operated by your ISP. These cache servers behave in unexpected ways
when transferring files greater than 255 MB in size. They can induce
corruption, or premature session termination.
The cache servers are usually transparent. You can't tell they are in
place until they corrupt a download.
Use a download manager when ever possible. Avoid downloading large
zips with your web browser. Configure your download manager to split
your download into numerous parts. Make sure that each download
segment is less than 255 MB. For example, if your zip file is 800 MB
in size, you need to split the download into 4 parts. The download
manager will automatically combine the parts into one whole file.
Not only does using a download manager increase reliability of
downloading large files over the Internet, they also increase your
We recommend ReGet as a download manager. ReGet supports SSL as well.
Do not try to resume a download on a different port then the download
was started on. Your computer thinks that identical files on different
ports are different files. If you switch ports, instead of resuming
the file, your computer will start the download from the beginning.
If you must use a web browser to download from the Zip Manager, make
sure the browser's cache is set to at least twice the size of the
largest zip file you plan to download. If you are using IE, make sure
to check both checkboxes under Tools|Internet Options|Advanced|HTTP
1.1 settings. You will have to restart your browser for these changes to take effect.
Make sure you have the newest firmware in your cable/dsl router. This
is especially important for linksys routers. Older firmware versions
contained incorrect MTU values that caused corruption at high speeds.
If you are unlucky enough to get a corrupt or broken zip file, use
PKZip to repair the zip and recover as many files as possible. PKZip
has the best repair algorithms compared to other utilities.