I have Snow Leopard 10.6.8 on my MacBook. Constantly dropping internet connection or message "error connecting to the network" comes up. No wireless connectivity issues with windows 7 laptop or iphone.

I have a MacBook with Snow Leopard 10.6.8. The last few weeks it has been constantly dropping internet connection, and one of these messages appear on the page: "Safari cannot open this page because the server where this page is located isn't responding" or "Safari cannot open this page because you are not connected to the internet" comes up. I will check Wifi and I am connected. No wifi connectivity issues with iphone or windows 7 laptop, this doesn't happen with either of those devices. I have unplugged and reset router, cleared cache, still happening. Any ideas? Thanks in advance.