Don't know the age of your machine nor in any changes over the years to how the PTO operates. On mine, there is an electrical operated valve that controls whether vac is applied to the solenoid or not. That part used to be an issue.
Seem to recall I replaced that valve once and problem returned. Next took the valve apart and cleaned it internally. Problem has never returned. Know of several others that also experienced the same problem as I was having. Following my advise their problem went away.
So my advice is to check the vac valve by bypassing it and if the problem is resolved, replace the valve or take it apart and clean. Next if that doesn't take care of the problem I'd start checking for leaks in the vac line and solenoid. There is also the potential for some of the mech linkage to be binding and I'd not overlook that if other items check out.