2005 Jeep Wrangler slow emissions leak help

Hello! I'm new here and really hope you guys may be able to answer a question for me.

Car: 2005 Jeep Wrangler 36,000 miles
What is happening: Check engine light comes on when fuel tank reaches around 3/4

Sometime this winter I replaced the original locking gas cap to a Mopar non-locking gas cap. Not long after I started getting the check engine light to come on. I have been to NAPA and Autozone to get it read and it says 'slow emissions leak', which typically means there is a problem with the gas cap. They have told me to tighten the cap and it will go away in a couple days. It will not turn off by itself, so I have disconnected the battery for a couple mins to reset it. The light will stay off until the next time my fuel level gets to about 3/4 and then comes back on. Is there something else that could be triggering this light? Or, why would an original Mopar part cause my computer to read an error?

Thanks for the help. Let me know if I left something out.
Sometimes the gas cap is not the problem. There could be a small leak somewhere else. Also the Jeep determines emissions leaks by applying vacuum to the gas tank and all associated lines. To do this there is a vacuum solenoid that only applies vacuum when the computer wants to perform the emissions test. There is also a vacuum purge solenoid that will release the vacuum when test is complete. The code is set when the computer doesn't see a predetermined amount of vacuum for a predetermined amount of time in the EVAP system(fuel system). So if the vacuum solenoid is not allowing vacuum at the correct time to the EVAP system or the purge solenoid is not holding the vacuum in like it should the computer will set the light because it is not seeing the vacuum it was expecting at test time.