OBD Scanner Tool Showing P0442 Code

My car is running fine but the "check engine" light, the VSC and TRAC control light all lit up back in the winter of 2015. My husband has the OBD scan tool and got the code P0442 which is some type of "emissions" leak. Took it to one shop in January with a supposedly certified Toyota mechanic; they suggested cheap fix of replacing gas cap. Replaced cap with genuine Toyota cap; reset codes; lights all came back on in 300 miles. Back to same shop, February, they suggested a diagnostic test and smoke test; found no leaks. They told me a "Computer Re-Flash" would solve the problem. Paid $158 for that; lights back on within 200 miles. April 2016, went to a Toyota dealership and they had the car for 3 hours; re-ran the diagnostic and a smoke test. They told me my charcoal canister had an "intermittant internal leak" and needed replacing. The price they quoted was outrageous so we bought the part from the dealer for $400 and my husband had another certified mechanic at a different shop put on the part. 100 miles later, those lights are back on and the same code, P0442, keeps showing up! We don't know what to do now; we reset the lights so I could take it through our state required emissions test--and the car passed! I have to do this test every 2 years here in Maryland. What else can be checked on this car? We can't keep throwing money at this car to "fix" the code. And we are totally perplexed because each time we were told the solution would "fix the problem" and it never has. Help!