PTO Problem Zetor 8111

Hi All i have just joined this site as i have looked at it for years and have always got good information from it.
I have an 8111 and the PTO is slipping on it.It turns on fine but when it comes under load it stops turning.When i put my foot on the clutch it starts to spin again.Has anybody ever seen this problem before

if i remeber right the shaft for the pto is direct drive from flywheel no clutch on it only in the pto pack theres dics that can slip .. this shaft all so drives the hydraulic pump for lift . a good test would be to get pto to slip and check the lift works .. .. but my gut feeling is your losing pressure in the pto back or low pressure this can be adjusted up maybe sum of the pro will confirm about the pto shaft is driving the hydraulic pump