Been there; done all of that. Let'* look at what happened to me.
I had a similar problem with my new 2001 Bonne after every fill up. The "check gas cap" light would go out after 10-20 miles or so, and return again after the next fill up. Mechanics tore their hair out, checking codes, testing cap, replacing cap, etc. Finally, one dude suggested that at fill-up I DO NOT OVERFILL BY ONE DROP once the auto fuel fill nozzel "cuts off". I thought the guy was crazy. Well, now I pull the nozzle out and cap 'er back up WITHOUT TRYING TO GET MORE FUEL INTO HER. I've never looked back.
2001 SSEi- black; stock 92K
1995 Vette; Competition Yellow; stock; 38K