2003 Rover 25 stalling long crank time

I have a rover 25 2003 model 1.6 lpg and petrol engine. It starts ok from cold start and also starts ok from warm start.
When the call stalls , it takes a long time to start 4 secs approx rather than immediate start. This length of time to start only happens when car stalls. I also stall the car quite often even though biting point is in the same position , I get the feeling not enough gas/feul is getting through. My engine light came on recently and it says cam shaft sensor or crank position sensor. Does these symptons described above associate with bad cam or crank sensor? Or is it something else. Thanks