The transmission on my truck slips when it goes into third gear.
There are a couple of possible issues. Looking at Ford’s technical service bulletins, possible issues range from water intrusion in a sensor to a possible warped separator plate between the valve body and transmission case to possible internal wear in the transmission. Fault codes will need to be read out of the Powertrain Control Module and automatic transmission fluid level and condition need to be checked to determine the proper path to repair. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the equipment and expertise to narrow down the cause of the slippage and recommend the proper course of repair.