Internet slows down during surfing in Windows 7?

[zitdzdtdj]

When I start my computer and I go to surf the web, I surf about 3-4 pages, and then my internet speed starts to drop very fast until it is barely possible to surf.
The only way to temporarily recover internet use is to reboot PC from scratch (disconnecting as user doesn't work). But even so, after a couple of clicks, internet becomes unusable.
Streaming videos or downloading becomes really frustrating.
In the safe mode the speed always is perfectly ok.

I tried using many different browsers and it's all the same. My pc is optimized, and bitdefender did not find any viruses.

your help would be greatly appreciated.