I am having a similar problem with my son's ATX after he ran it on a ballfield last weekend. We have been there several times before, but last weekend it was pretty dry and dusty. It started running really rough and got real hard to start. I took the filter off and found some dust had gotten past the filter. Last night I took the motor partially apart in fear dust had scratched the cylinder up. But it was good. I do not have a problem with the fuel returning down the line, but sometimes when I try to start it I cannot get fuel to go into the cylinder. I can cover the exhaust and pull it 15 or 20 times and it will not load up. So I think my problem is either a clogged up carburator or an air leak somewhere causing it to run lean. I took the adjustment screws and filter off the carburator and cleaned it out pretty good. I am going for new fuel line tonight to see if that fixes it. Also, my problem is worse when the motor is hot unlike yours. I would start with new fuel lines and look into the carburetor to see if anything is loose or clogged. Just my 2 cents.