The only thing I can think of would be some sort of file restriction by your ISP/Website provider. Is your website some sort of free one, or given to you by your ISP? I've never heard of problems like this before, and you said that you've had problems in the past with large files.
You can always try the default answer - connect your PC via wire to your modem. Power off everything for 10-15 seconds, power everything back up, and try again.