Well... I checked the plumbing inside the tank and it looked fine. So I took it back out to the field to give it another whirl... and it ran better. Not perfect, but better. I was able to fly several flights without it quitting on me. It surged and sputtered a few times, but I was able to wiggle the throttle just enough to keep it going. Unfortunately, I don't have enough confidence in the motor to fly the plane like I'd like to.

I'm baffled by its behavior.