Hello, I have a Proform Spacesaver PTFL21460. After walking for about 30-45 seconds, the walking belt stops moving. Get off the belt and it spools back up again until it quits again, 30-45 seconds later. The walking and motor drive belts appear to be adjusted properly, and I have lubed the walking belt. How can I diagnose this? What could the issue be? Thanks in advance for your help. Dave.