How can I fix my Mercedes 2300 sport estate car?

I drove my car yesterday and it did not seem to have any power went I put my foot on the accelerator, but if I lifted my foot off the accelerator it picked up speed. It was if there was dirt in the carburettor on any other car, only my car does not have one. I called the AA who cleaned the throttle cable and although it was a little better, the chap said to take it to a garage for a diagnostic test. Any ideas please. Many thanks.