treadmill belt slips right when running but left when no one is on it?

I have a Trojan treadmill and when I am walking like 5.5 Km per hour the belt is ok but when I run like at 7km per hour, the belt slips to the right within a few minutes. The belt slips to the left when the treadmill is running and there is no one on it.
What should I do please help
Doreen, January 2011