Speed limited to 33mph

Whilst driving normally with no problems the car suddenly lost power and would not drive above about 33mph independent of which gear was selected or depression of the accelerator pedal. Up to that speed the car would drive absolutely normally. No warning lights were activated and no faults reported. No speed control/limiter stalk is fitted just the one that controls the lights, indicators and windscreen washers. The AA patrol man could not correct the problem or identify any problem using his diagnostics computer. As the car was on the motorway I had to be towed home. The next morning the car drove normally with no warning lights or messages however I can't get it into my local merc garage for over a week. Any ideas please as to cause or how to correct it if it re-occurs? Could a faulty battery be a cause. Brake lights are working normally.