To start with the easy stuff, check the alternator output voltage. It should be around 26volts. That machine only has one computer, it controls your transmission and electrical operated grapple functions. I can't remember the exact number but if the voltage supply to the VCU drops below a certain voltage the transmission controller will kick out . I seems from here to be electrical as well. If the internals we're slipping it would just remain in gear on the monitor. Next thing to check is the wiring going through the center to the rear control valve and make sure it has not rubbed on the drive shaft or been damaged by a stick. Next thing is brake pressure. The park brake is spring applied and hydraulically released. If the park brake pressure drops below threshold it will kick the transmission into neutral. The Park brake pressure sensor is located on the right side of the hydraulic tank where the lever goes through around that tangled mess of metal tubing. Also pay attention to the service brake pedal to see if it has any resistance when it kicks out.