I'm getting a problem that is similar to yours, except I'm using a laptop and my wireless modem connects directly to the laptop's USB port.
I'm thinking it's switching between GSM/3G, because whenever I switch to GSM, almost none of the pages I go to will load, and if they do, they are super-slow to load, even when they are very lite. But on 3G, it's MUCH MORE faster and experience almost no problems, except for my connection dropping out whenever I do something that hogs my data allowance.
This is just my opinion, it could be something else.