Internet disconnects.

Hi, I recently reinstalled my macbook pro with a snow leopard disc. everything went well, but after a moment I lost my connection to the internet. everything looked like normal and the network showed that I had connection, but I didn't. I found out that if I clicked the disconnect button, and then tried to reconnect it would work again for some minutes. I want lion, but because I dont have app store I cant download it, and to get app store, I need to update my mac, but I loose connection, what should I do?