03 Mustang GT Automatic Stalling

"Non stock intake" - any time you have non-factory components, especially when you personally can't validate how it ran before then intake was installed -- anything goes. It's best to restore the vehicle to factory condition and recheck operation. If the problem still occurs, you've eliminated the variable and can continue testing otherwise. Trying to get into the transmission as a possible cause of concern without addressing modifications allows for too many variables. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops