Q: Slip's out of gear

If the filter has never been changed you could try that. When cold, debris has dropped to the pan. Once you start the vehicle, the filter screen can get blocked as fluid is drawn into the filter by pump suction. Of course, if the filter screen is blocked, there will be insufficient hydraulic pressure and the transmission simply will not work. Apart from a blocked filter, there are two main categories that would be diagnosed: electrical controls, including the transmission control module and the shift control devices such as solenoids, and the hydraulic control system, particularly the pump and valve body. With regard to the hydraulic control system, heat could even be causing the valve body to leak if the valve body is loose or distorted. Those sorts of issues are detectable by looking at hydraulic pressures (via external gauges) as the transmission runs. Based on what you are describing you will probably see one or more gauge readings that are way out of spec thus providing a clue as to where the fault is. You could monitor these hydraulic pressures when the transmission is cold and see how the pressure changes at the point at which the gears drop out. If you desire that a certified mechanic identify the root cause(s) of this failure to engage in gear and shift, simply contact YourMechanic and request a transmission diagnostic.