Also, LOL at California. The TB in the link is the STOCK FACTORY GT500 throttle body assembly. But because its being sold through Ford Performance brand it has a different part number..... and because of that it does not pass CARB EO certification.The GT500 is picky about throttle bodies. Stock OEM is always good to go. Ive run a VMP twin 67mm no issues, Whipple Oval Monoblade no issues with the Whipple needing to have electronics swapped. I have a KB 168 Mono that needs to be rebuilt and an L&M Twin 72mm that I have never tried. Bought it cheap as a project, but they were either the cats ass or a paper weight like your BBK.
Twins bigger than 67 seemed to have have issues for whatever reason
Either way, you'll want to pick up an OEM throttle body assembly. LMR has em pretty cheap:
Ford Performance Mustang Shelby GT500 60mm Throttle Body (07-17) GT/GT500 M-9926-MSVT
Install, and drive car. If same condition remains then you have an issue elsewhere. Im confidant though that its just a poor TB.
LMAO, what a shit show.