My car is having the same issue. The shop told me (a long time ago) it could be the pressure sensor on the gas pump but I don't think that is it.
EVAP system in stock form has....The EVAP purge at the front in the engine bay, charcoal canister on the driver side rear behind the tire under the body of the car. There is a sensor connected to the charcoal canister that opens and shuts the purge for the canister. Right now I don't have the front I cut that off and put a little filter on it. I plan on removing the charcoal canister and seeing if I can just spice the two lines together.