Lift slow and eventually stops as machine warms up

I've been trying to diagnose this one for a while... Bobcat 753G

I now have a pressure valve on a tee on the retraction side of the lift boom - lowering the boom with the bucket on the ground gives me:

(With the machine cold), I get 2100 PSI at idle and 2500 PSI full throttle
(With the machine warm), I get around 1100 PSI at idle and 1500 PSI at full throttle - when the machine is warm it isn't able to lift the boom except for small jerks when I press and release the pedal

Tilt seems okay...

Lift slow and eventually stops as machine warms up