I think you are describing what we call "Limp Home Mode"

There are many threads on this site describing causes. As I recall, the most common things are:

The Brake Light Switch
The Over Voltage Protection Relay (I think that is what it is called)
The Throttle Position Sensor (I personally experienced this one)
The Engine Wiring Harness

There are, of course, many more possibilities...

Is it intermittent? Does it happen when you tap the brake? There are a few subtle clues to observe, but it can sometimes be a pain to cure this.

Good Luck!

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