i had the stock throttle body on my car, it would rev up to 2300 by itself and stay there, if i shut the car off and turned it back on it would be fine for a bit then rev up again, i replaced the throttle body with a bbk 75mm, was fine for about a month but the past couple days its been reving to 1500 by itself and staying there, anyone got any ideas on what the problem could be?

1991 5.0 mustang gt