Q: Vehicle shifts down when traveling 55mph+ when gas is not being pushed

Shift points are controlled by vehicle sensors such as the vehicle speed sensor and throttle position sensor. In addition there are shift solenoids in the transmission valve body and, in the scenario you are presenting, torque converter lock up controls. What you are describing is a premature shift out of overdrive when the torque converter is locked up (based on the vehicle speed you state the fault occurs at) but really any of the foregoing components could be implicated. All will have to be tested. The transmission is electronically controlled so applying a scan tool will provide some clues as to the origin of the fault. The brake issue is probably due to air in the brake lines but bulging rubber brake lines would be considered, too. Brake fluid should be exchanged every 3 years or so to eliminate moisture which, upon boiling, can introduce air into the brake lines. Loss of power is often due to plugged fuel injectors but there are other causes. Given the age of the vehicle, the injectors should be removed from the fuel rail and sent to a lab such as Mr. Injector for ultrasonic cleaning and filter replacement (each injector has a mini filter within that has to be replaced and that is why on the car cleaning is pointless). To have all of the issues you are writing in about evaluated I would recommend just booking one service appointment. The mechanic will come out and evaluate what you have and give you a least cost strategy to get all the repairs done at one time. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.