Internet sometimes very slow on my laptop

Whenever the internet on my laptop slows down everything is slow. All 3 of my browsers (GC, FF, IE) slow down and so do my download managers. When it is slow it is slow for several hours and restarting the computer doesn't help. If I leave the computer on all night and then restart it in the morning it is usually back to normal. On my other laptop the internet is fine. My internet is 6 Mbps DL and when it slows down it is about as fast as dial-up. The problem is the same whether wired or wireless.

Thanks for any help

I did a clean install and I have TuneUp Utilities. TuneUp Utilities 2010

I did a complete anti-virus and anti-malware scan and nothing. I have always had my BitDefenter tuned on.