Internet is working but keep getting ISP failure

I have a macbook pro, 10.6.8., Internet etc. are working fine. What happens is that if I quit safari, or the computer goes on standby, it suddenly won't connect to the internet, I go to network preferences, assist me, diagnostics, and it will briefyl have ISP/Internet/Server in yellow or red, when I click through to have it check the connection everything goes green again and the internet works as normal. One of the setting's etc. must be off, but I don't know how to fix it, help?