Internet is suddenly dead

Hi

I'm using an essential internet service from the last month. It is not pretty much fast but generous speed to check email and to watch some youtube videos. However, when I upload some files, even a single small size image file, I cannot use internet during that time. If I open a web browser at that time, it cannot open any website and displays that  internet service does not respond. I also returned the modem and replaced another one, but there being same situation.