why does my treadmill slow right down when I step on it tx400 model #04040?

I have a tx400, My problem is when I get up to the speed I want and step on the treadmill it slows right down to crawl. I have tried everything I could to see if I could fix the proplem myself to no avail. Now I can get it up to full speed and it runs like charm until i step on it, then down she goes. Is there somebody out there that can help me.