Re 784 PTO, I assume you mean the PTO slows down or stops turning when under heavy load?
Sometimes it is caused by the control lever linkage becoming seized up with dirt and rust.
Just clean out area and apply your favourite penetrating oil while working control lever.
Other times it is caused by lack of hydraulic pressure to the hydraulically activated PTO clutch (220 to 250 psi). This can be caused by clogged hydraulic filter or MCV valve or weak hydraulic pump. There is a pressure adjustment but requires special tools.
Do not operate the PTO with it slipping or you will burn out the PTO clutch.
Simple things first, clean and lube control arm mechanism, check hydraulic fluid level then replace hydraulic filter.
My experience on these tractors if there is a problem with the hydraulics the first symptom is loss of prime to the brakes. Requires constant bleeding of the brakes. Does the parking brake work? or is the remnants of the parking brake band floating around in the hydraulic system?