Peugeot 406 diesel keep revving by itself?

If it's pulsing the revs - jumping up a little over idle like you're just touching the throttle - that could be a bad injector not spraying properly and giving you uneven running. Sometimes the revs can jump as high as 1500+rpm, and it's worst when very cold or very hot. Another symptom if it's that would be trouble starting it in the cold, no matter how many times you run the glow plugs.