As I understand it, once the canister is clogged it is done, and drying out doesn't help. That may or may not be correct as I have not experienced it.
Moose: If you weren't seeing the fuel backing up and then not backing up I would advise that you should use the same pump every time if you are trying to find out what is causing the problem. A different pump, even at the same station, can be set for more or less back pressure.
Since you can actually see a difference in how the fuel is going into the tank I am more inclined to think it is the car. Maybe the vent solenoid valve is intermittently getting stuck closed, since I don't know what else could cause this. (You are turning the car off to fuel it, I am sure.)
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked