renault megane 1.6 vvt running problems?

hi i have a renault megane cab 1.6 16v which has running issues, when warm it sometime misfires for about 2-3 seconds then keep going as normal, now it cuts out and stops, hit the start button and of it goes o.k. until next time?, reading the codes it comes up with cam position sensor of which it now has had 2 and a crank sensor but still has the problem, now am just about to replace the de-phaser pully, water pump and cam belt kit hoping this will cure the problem! - does any one have any idea's why it does this???????? - thanks mark.