Wet (water in the) gas can make it act just like this. Try another tank of gas after you run this one all the way out, or just drain it and start over.
I don't know about other areas, but here in Denver, CO, the "economy" gas stations are running poor maintenance on their tanks because of slim margins on the gas. They are not shutting off the pumps early enough, and the water levels are too high in their tanks.
I recently went to a cut-rate station because I was out of gas and it was there, and in 3 miles the engine would not idle, despite the fact that the carb was overflowing onto the intake manifold. There was so much water in the gas that it was separating in the little puddles in the manifold, clearly visible. The float in the carb would not close the inlet valve because the specific gravity was so far off.
I bought 6 bottles of methanol ("gas line anti-freeze" they call it) and limped through the day, drained half the gas that weekend and added more methanol and good gas, got through it. Same weekend, our neighbor's daughter had to have her car towed home after filling at another similar station: Dad was draining the gas into buckets and it was obviously more than 50% water.
The stations always have water in their tanks: the fuel pickups in those tanks float on a little bouy that is supposed to get just the gas on the top. When the gas level drops near the water level, which is monitored by a special instrument that can tell where the water level is, an alarm light shows up for the gas station operator to go out and put a sign on the pump that says "Out of Gas". Trouble around here is, the operators don't give a $#@! and ignore the warnings, particularly at the cut-rate places.
When the water gets so deep that the station cannot hold much gas, they call a service that comes and pumps out the tanks from the bottom, so as to pull out the water. On those trucks, there is another special instrument that indicates the amount of water vs. the amount of gas that is in this mixture: they stop pumping when it eaches about 50/50. This leaves about 1 foot of water in the tanks. This is a "blanket" that will leak first if the tank starts to leak, so only water will leak into the ground from the bottom of a tank. There is another indicator in the station's system that keeps track of tank levels against how much gas is pumped: if the level is lower than expected, it sets off a leak alarm and the station shuts down, calls for a service to come out and remove all their gas and water, and they dig out the tanks to start over.
Simple, huh?
(BTW, I'm an engineer, and I've designed this crap before, that's how I know....)
