Water in the gas? I've had it before. It was most noticeable in the morning when pulling away. It would run great for 1/4 mile, then sputter for a bit, then be mostly ok. I suspec it was running on what was in the line, then sucking through the water that had pooled in the bottom of the tank overnight.
Just get some gas out of the tank and into a glass jar. Let it sit for a while and see if any water droplets form in the bottom.
Scott