John Deere 3140 Hydraulic problems

I have a ~1981 John Deere 3140 with hydraulic problems. I have replaced the filter and cleaned the screen and replaced the oil. The spool valves are generating about 180-200bar (2800-2900psi) at idle, my gauge was only rated to 150bar so I've a larger one on order. The pressure is constant and instant. The oil level is quite low, below min mark on the dip stick, so I can only assume the pumps are working well. The problem is the 3 pt linkage shudders when lifting, at idle speed the steering doesn't work properly and needs higher revs. During a couple of hours work lifting anything heavy the oil almost boils. The 3 pt linkage will drop a heavy load unless the revs are flat out and especially when you use the hi lo changeover in the gears. When I listen in the filler cap I can hear liquid spitting inside. When the tractor is turned off the 3 pt linkage drops in about 5 seconds with a load on behind. Is there any way of isolating the problems to a single cylinder which is causing the problem