P0446 and P0401 Fault Codes on 2001 Hyundai XG 3L V6 - Causes and Solutions

I'm seeking clarity on two intermittent fault codes appearing on my 2001 Hyundai XG with a 3.0L V6 engine and electronic 5-speed automatic transmission. The first issue is a recurring P0446 code, which appears during long downhill descents—such as a nearly half-hour descent of over 2,000 feet—where the Check Engine light activates. After clearing the code, it remains off until the next similar driving condition. I'm wondering whether the sustained negative pressure in the fuel tank during steep descents could trigger this code falsely. Additionally, I've experienced sporadic P0401 codes on the same vehicle, though not during the same conditions as P0446. These occurrences are random and unpredictable—sometimes appearing after a cold day or when temperatures drop. Once cleared, the code may stay off for anywhere from one day to six months. I’ve already inspected all vacuum hoses and disconnected all electrical connections related to the evaporative emission system components, but no consistent pattern has emerged. The codes do not affect drivability or fuel efficiency. However, given their frequency during colder weather, I suspect environmental factors may play a role. While the HMA service manual is extensive, its detailed troubleshooting steps seem impractical due to the randomness of these issues. Are there any known solutions or diagnostic tips for P044 or P0401 on this specific model? Any insights from owners with similar experiences would be greatly appreciated. Thanks, B Sea, British Columbia, Canada