I created a custom setup for the XF GTI and now the AI drivers don't know how to drive them. They're fine for the first 10 seconds, then they give up and stop acceleration, driving only until they run out of momentum. Help!

perhaps they dont have enough fuel ?

Never seen such thing. Care to post the setup?
PS. Fuel should not be problem as AI picks it itself,you cannot influence it.

Maybe some strange gearbox settings that AI drivers do not like and burn the clutch?

Can you post the set so we can have a look?