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Ideally the garage has to plug the car in and try to replicate the issue that your seeing and capture the event or codes that are stored..to me going by what your saying i maybe leaning towards a faulty brake switch,it doesnt just activate your brake lights..it also send info to the ecu when your slowing down.trim fueling..so might be worth looking at that.wiring to it...
It still possible that the switch is at fault albeit intermittent..when you coming to stop or full stop or pressing the brake pedal the gearbox is actually kicking down obviously,to first..unless you have an issue in the valve block on the gearbox..it difficult to say over the 2 but the brake light switch issue and then the fault recitifing itself after ignition reset is quite common on the ML's thats why it best to inspect the switch on the pedal box,wiring to it,or get it on live data or have the mechanic drive the car,whilst its plugged in to capture the fault..
Its still possible yes for the car to be in limp mode/stuck in first gear , until your reset with ignition... before you actually by another switch..it would be best advised to have it diagnoesd properly and confirm the fault...
Your welcome..if you need anything else or need advice..then just ask..