Our Kenmore model 721.80833500 tray jumps off its track. This has progressively gotten worse over time and now the little end wheel on the rotating arm has broken off. The microwave still works and still jumps off the track. This jumping off the track with the tray has been occurring over the last year or so and now it happens more often. At times it appears that the heavier the item being cooked the less the tray jumps off the track. However, this too is intermittent.