what causes my car's transmission to go out of gear when driving under 40 MPH

This could be electronic if it's completely coming out of gear while driving. If you are able to temporarily remedy the issue by turning the car off and restarting, I'd bet it's electronic. This would require at least a good scan tool to get started. Live data will help greatly. If you don't work on your own car then use this link to find a good shop near you. https://repairpal.com/repair-shops Alex