Need answer to this issue

When i watch a movies or open a game, such as Warcraft 3 after 10~20 ou maybe 30 min then i start to lag a lot.. and it stop after like 2~5minutes and then..again ..none-stop lagging randomly

I formated my whole computer, and run Anti-Virus/Spyware .. Defrag and all...
still same result..

So i think is my Wireless thing that are maybe something wrong./ nor some1 stealing conections, maybe?

PC spec
Linksys.
2gb of ram
E-Geforce 7600GT 512mb
Pentium D 2.8
Window XP service pack 2
DirectX 9.0

And another thing, i had 1gb of ram before with 2x512 and it dosn't happen to lag like this..

Thanks