I don't want this to sound offensive.
My 140 does the same thing whenever the gas gets low. I don't mean that your tractor is out of gas. I mean that on my tractor the when the gas gets less than 1/2 full, that it starts to run bad. I guess this is a full pressure problem from the gravity feed system. Keep the tank as full as posible, this may help. A full tank will put more pressure on any trash in the line and will maybe push around it or push it on through.