Q: when under load the 2003 3.8 v6 mustang doesnt accelerate and the transmission feels like its slipping

If the transmission clutches are slipping, that is an internal mechanical fault that typically requires re-building. If the issue is in engine controls though, other common causes of loss of power include plugged fuel injectors (all should be removed from the fuel rail and sent out to a lab such as Mr. Injector where they are cleaned and restored), electronic throttle controls, leaking air intake hose, faulty fuel pump or restricted fuel system, and large vacuum leaks (including EGR malfunctions, PCV system, leaking intake manifold, leaking EVAP purge valve). A scan tool would be used to look at real time conditions within the engine (particularly short term and long term fuel trim, etc.) and identify the root cause. If you want the required diagnostic tests performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a loss of power inspection/diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.