Your car IS going to want to die at 300 RPM, but that step-up selonoid should compensate for an RPM drop in any situation. Either Bama messed with it or it was faulty before the tune and now the tune is the last straw.
2014 GT Premium. Cobra Jet Intake with monoblade throttle body and FRPP CAI. Ford Racing Sport axle backs. Pypes off road H pipe. Numerous appearance mods, it was a plain blue Mustang when I bought it.