Gas boiling

I had the same problem with my 66 with a 273 added a 1/2 inch [phenolic] spacer it cured it,but now i'm having issues again the spacer is about 2 1/2 years old could be soaked plus the crappy gas, i'm buying non ethenol high test here tried a small amount last time but this time i'm gonna go for a full tank see what happens.