Does it smell like it's running rich when at lower rpms, just before it dies? If it's a quadrajet, sometimes during a rebuild the primary metering rods get bent durning reassembly, so they aren't in the jets restricting flow. If that makes sense. When you take it apart, it should be very obvious if that's the problem. We stopped and looked at a boat on our way to Rathbun this year, and it was having the same issues.
Terry