bmw 318i 97 revs itself when hot and stays at 3000rpm?

hi i have a bmw 318i 1997 the car runs fine when cold untill it reaches normall running temperature it revs itself up to 3000rpm by itself and wont drop again and when you put your foot on the accelorator it splutters and cuts the engine out, then wont start again for about 20 minuites, please if anyone can help will be very gratefull.