Not sure I can help much here other then give some ideas.
To me it sounds like something going on with the valves. Something sticking somewhere. Maybe a bypass valve stuck open.
When you say tilt the bucket up and the lift up works better, are you holding the tilt up peddle all the way down? This would cause a bypass to open when the bucket is tilted up all the way, since it has nowhere to go. Not sure if this bypass could help charge the lift pressure.
Does the Machine have the option of bucket possition valve? Not sure what would happen if something went wrong with that valve. I mention this because of the tilt peddle "fix"
Long shot…Not sure of your machine, specifically since it had the option of several different controls, but make sure you didn't somehow turn on the auxiliary in the lock open possition. This is pretty hard to do with electronics, but easy with manual controls.
Hopefully someone else on here can give you a more specific answer.