Boom won't lift

we have a Timberjack 608 track feller buncher, which I could of posted this in the forestry section but a feller bunch has the same operation as an excavator, from time to time the main boom cylinder will not raise. It will lower but pulling on the joystick it will not raise the boom then in 10 seconds or sometimes longer holding on the joystick it will finally raise & then be fine for 15-20 minutes & then do it again. Seems to happen more once oil is warm. Anybody have any suggestions on...

Boom won't lift

Boom won't lift