Wow, good report on the symptoms and the attempts to resolve. It sounds to me like you have an issue with oil control valve.
Make sure the lift locking/unlocking bolt is closed. I doubt that is the issue but can cause loss of lift efficiency. Confirm the lift spool is stroking completely. With the aux functions being normal, it doesn't sound like a pump problem. As far as barely lifting, tells me the oil is not being solely directed to the lift cylinders. I would think you could tell by feel if there is a significant flow through the return filter or hoses.