Discovery II – Lose strength and the acceleration

See if you can help me:
I have my Land Rover Discovery II – 2000 experiencing power loss in normal driving without abuse. Everything starts without problems and at a certain moment between 2,000 and 3,000 rotation it loses strength and the acceleration with great difficulty increases, I get the feeling that the car changes to 4 cylinders and with some vibration.

The strangest thing is that it just goes back to normal, this is repeated even in small laps with the car.

What do you think it might be? Thanks