I think your right when you suspect a fuel delivery problem. since it was scanned and didnt read any codes suggesting ignition problems, I would think the next possible thing would be fuel. cant be timing because its an intermittent. A wild guess... since it drives well in city miles, where the gas is being used on and off, the fuel pump or maybe just a clog somewhere is preventing enough gas to be injected into the engine when your wife keeps the pedal in a constant place. but like you said as well, if its fuel it could be a whole buncha things. (gas cap, filter, pump, etc...) good luck, youve got my attention on this one. as much as I cant stand or trust some dealership mechanics, i agree with charlieb, let them have it for a while until it happens to them.