Sounds like one of these issues.
1. the fuel pump isn't keeping up with the demand
2. vacuum build up inside the tank - make sure the fuel tank is vented or use a vented cap
3. the fuel bowl float needs adjustment.

I suspect #1 or #2 is the problem. It take a while for the fuel tank to build up vacuum. Drive it until it sputters and then stop and pop off the fuel tank cap. If you hear suction then you may have found your problem. Especially true if you can then drive it again without sputtering for another 30 minutes.

Let's see if others agree.