Hello, We have a 2006 Toro Z400 48" mower with 23HP Kawasaki. Today the deck belt started derailing itself for no apparent reason. We put it back on but continued to happen again and again. The right side (discharge) idler pulley closest to front of the deck was a little loose. We retightened and reinstalled the belt. Problem persists. At the advice of my dealer, we started looking at the idler pulley with the tensioner spring (left side, slightly forward of rear tire). We replaced this idler pulley with another identical brand new one. All other parts seem secure and working properly. Problem persists. Please Help!