barkley

Hi
I have just bought an X reg 206 and have noticed a slight problem.
At random intervals the rev counter stops working. When this happens the engine noise seems to increase dramatically.
Then the next time you come to firing up the car, the rev counter begins to work again and the engine noise is back to normal.
I would compare the noise of the engine to be similar to the sound of a school bus when the rev counter stops working.
Has anyone had this problem or does anyone know why this may be happening? I thought the rev counter was purely an electrical component, so how can this relate to the engine noise increase every time it stops functioning?
Thanks for any help you can give.