Windows 7 - Using any browser my download stop at random

Hello,
It's my 1st post, and english isn't my 1st language so please be gentle...

I have a very weird problem.
I have a fresh windows installation (4 days) using windows 7 ultimate x64.
I use google chrome. Today when i wanted to DL a file, at random moment dl speed has went down. 1st it showed normal dl speed (2mb/s) but the % of download and time to finish download didn't changed. Then it goes down to some funny number lik 20 kb/s...
i downloaded mozilla firefox, opera and even tried IE, but the same problem occurs.
I disabled Kaspersky internet security 2012, but this didn't work.
but the weirdest thing is that speedtest.net show normal values for download upload and ping.
Well sorry the weirdest thing is that i even downloaded "free download manager" and it downloaded all of the files (tried 5 different from different sites) with no problems, always above 2mb/s.
I am using linksys router so i unplugged it and plugged directly to modem, didn't work
I have preformed virus scan, with no results.
I did not change any registry entry.
The download stops at a random moment, each time different than the other.

I have already tried solutions posted here:
Internet speed suddenly slow?
All web browsers stop downloading data every few seconds
none of them helped.

PLEASE ANY HELP WILL BE APPRECIATED

Udpate 1:
Seems that download manager also have problems, but at least after it stops downloading it resumes (but with lower speed ~800kb/s) after a while

What are you trying to download and from where? What programs are you using to download with?