Those chains need frequent adjustment as the 7400 streses them. Drivers picking up double loads wont help either,
The main lift chains need to be tight with tellies down. There should be a gap at top tellie. If its nested all the way down, with no gap, stress is put on main lift cyls. They can cause cracks at base of main lift cyl mounts.
If you get intermittant codes 64 or 67 and have slow travel, check tellies to make sure they are nested all the way down. Sometimes a main lift cyl cushion spring is broken, causing the cyl to hang up one inch. The prox at bottom wont see the tellie, causing codes and slow speed.