It’s not sporadic. It just has to fail the self test X number of times before it will trigger the CEL. I bet if you reset the computer and drove it twice or three times the EVAp code would show up as pending. That means the computer has detected a problem, but it hasn’t yet confirmed it as a chronic problem.
My course of action would be to buy a new gas cap (cheapest option) and if that failed I would either take it to someone, or get a service manual (probably take it in, cause of the SKIM problem).