It sounds like the intermittent "Reduced braking power" message and ABS warning on your E350 could be related to a few potential issues. One common culprit is the brake booster or the electronic brake control module, especially given the age and mileage of the vehicle. Problems with the brake booster, such as a vacuum leak, can reduce the boost assist to the brakes, leading to a loss of braking efficiency and triggering warning lights. Additionally, faulty wheel speed sensors or connections can intermittently disrupt the ABS system, leading to erratic behavior and warning messages. Electrical gremlins in older vehicles could also cause communication errors between the ABS sensors and the car's control units. To dig deeper into the issue, I recommend checking the fault codes from both the ABS Control Module and possibly the Brake Control Module. These modules will help isolate whether the issue stems from a sensor, a wiring problem, or something more severe like the pump or electronics in the brake system. Using a comprehensive scanner like the youcanic full system scanner will give you access to all these relevant diagnostics. Keep in mind that a basic code reader, such as the Bafx OBD2, might only show generic codes and miss detailed brake system faults. Have you checked if the brake fluid level is adequate and inspected the condition of your brake pads and rotors? Sometimes, these simpler checks can reveal underlying maintenance issues that contribute to warning messages.