Data Network Issues

I have the iPhone 4 16gb. I'm with the Optus network. My sister has my old iPhone 3gs, also Optus network.

From the hours of 7pm to 11pm I have trouble connecting to the internet. However hers, its perfectly fine.

At first I thought it was Optus, but after running through everything with them, I'm still having trouble. Could it be a phone issue? I was told I can have it replaced from an Apple store as its under a month old but I won't do this if it isn't a phone issue. He couldn't seem to figure it out seeing I had my phone and my sisters side by side and her works perfectly.