Weird network problem

Greetings everyone!,

I'm new to this forum but I found it amazingly helpful; once more I seek answers here but my question is regarding a strange problem I'm having and seems impossible for me to fix.

About two weeks from now I started experiencing an incredible decrease in my internet speed, but it results in a weird set of facts: when I visit www.speedtest.net to test my internet speed it shows the correct speed its suppose to show (the one I bought to my ISP) but when I download files most of the time the speed decreases 80-90% and its extremely annoying! websites load perfectly fine (most of the times! not always) and things like YouTube videos load very (very) slow.

I called my ISP obviously and they said everything was OK. So I thought it was my Wireless Router; I disconnected the router and plugged my computer directly to my ISP's modem, didn't work. Then I thought it could be my computer, I tried the other PCs at home and even brought a friend's Laptop and didn't work.

So I'm practically desperate right now and I don't know what to do, my last hope is to call my ISP and make them come over to see if there is some sort of cable connection problem but I don't think so I mean, why would www.speedtest.net (and several other speed test sites) display the correct speed and I experience different when I really use my internet?!

Let me know if you need any info on my settings and I'll provide them to you.

Please help me!!
Thanks!,
Xflash08.