Fuel spillage

I am having a problem with my 64 Valiant, when you fill it up it does not spill out of the filler neck, but it does so after you have driven the car, you park it go in the store come out to find it is puking gas out the filler neck, it has ruined the paint it does it so much, I was thinking that there was something wrong with the vent tube, but can't find any problem with it. I have looked at where the cap threads to the filler neck and it looks ok, I have put a NOS gas cap on it seals good, I still get fuel spilling out. Has anyone experienced this if so what did you do to solve it. I am getting frustrated with this problem, as this is my girls daily driver. Thanks in Advance........