Bingo! Good call! The purge valve was indeed the problem. I had been planning on taking the car back to the shop today (based on my family and work schedule). As luck would have it, the SES light came on the day before, and codes pointed to the purge valve. Everything good since I picked it up today. I had been suffering with this for the past 5 months.
If you top off past the vapor return line on top of you filling tube, the overflow will make its way to he charcoal canister and plug your vapor return line.
Sorry for joining in late, but I had to change that valve on my car about a year and a half ago. I had the same symptom you described. Glad it's fixed!
Gosh, this was the very first reply to the thread and it was right all along. At the time, I didn't understand what a gas tank breather (aka purge valve) is or why it might be related to the issue.