incline motor slips with weight on it?

True 500 SL works well, but when I incline the tredmill, it will incline to the selected position,then it will sound like it adjusting dowwards (Motor sound), stops and immediately goes back up as though it is adjusting for lost height. It continues to adjust down, then up, as long as I stand on the unit. I can get off, it will stop, and will maintain the vertical position, even when I get back on. Any ideas? It has a couple of microswitches that will serve as an upper limit and a lower limit.