Skytrak 8042 legacy, ocassional no boom down

Hello to all, New to the forum, hope someone can help. 2006 Skytrak 8042 will occasionally lock up on lowering the boom, increasing rpm will force it to lower but engine is under a lot of load. Turning off the engine and restarting will free it up and it works fine until the next time it decides to lock up again. We are looking at possible counterbalance valves and or pilot operated check valves located on the boom lift cylinders. Any help much appreciated. Thanks