Hi I have a year 2000 bmw 328ci auto when I start

Hi I have a year 2000 bmw 328ci auto when I start it but only sometimes it takes 3 -5 turns to start usually takes only 2 turns to start when it takes 3 5 turns I have lack of power and some times it stalls at traffic lights etc when you put your foot down it feels like its struggling and only moves off and accelerates slowly I have changed the cam belt sensor and the crankshaft sensor but it still does it. it not when its cold or hot it does not do it all the time what I have to do is to stop the engine and re start it again then its OK? any ideas?