Losing ability to download on iMacs.

Question

We have a strange problem here at work. We have a combination of iMacs and PCs and we're having network issues with the iMacs. On the iMacs anytime someone tries to download a larger file (i.e. tries to watch a video on Apple.com) it downloads about 200MB and then loses the ability to download for a few hours. It doesn't loose connection just losses ability to download anything. Waiting a few hours or changing the IP address will bring back the ability to download but will have the same issue if you try to download a large file again. Small files are fine. We can surf the web all day without issue. The PCs have no issue at all.
Ideas?
Thanks!

Answer 1

Please read this whole message before doing anything. This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it. Step 1 The purpose of this step is to determine whether the problem is localized to your user account. Enable guest logins* and log in as Guest. For instructions, launch the System Preferences application, select Help from the menu bar, and enter “Set up guest users” (without the quotes) in the search box. Don't use the Safari-only “Guest User” login created by “Find My Mac.” While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin. Test while logged in as Guest. Same problem? After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it. *Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing. Step 2 The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login. Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode* and log in to the account with the problem. The instructions provided by Apple are as follows: 
Shut down your computer, wait 30 seconds, and then hold down the shift key while pressing the power button.
When you see the gray Apple logo, release the shift key.
If you are prompted to log in, type your password, and then hold down the shift key again as you click Log in.
 Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs. The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin. *Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t boot in safe mode. Test while in safe mode. Same problem? After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of steps 1 and 2.