Clk 230K Looses Power at High Revs

Hi Guys,

Ive got a 1998 230K CLK Convertible.

When I drive her normal there is no problem, but as soon as I put my foot down I loose power and the revs drop, if I take my foot off the throttle and slow down a little the power comes back as she drives as if nothing has happened, if I take it easy with the throttle I can get to over 100 MPH so long as I dont put my foot down.

Any Ideas what might be causing this.

Thanks Guys

Ray