Intermittent power loss when driving

Car Intermittently cranks for few seconds before starting then when driving will intermittently lose power and won’t go over 3,000 to 4,000 rpm's. Had check engine light come on showing cam position sensor, changed it few days later same symptoms RAC replaced sensor for second time and few days later symptoms returned. Garage said cam timing chain and tensioner needed replacing this was done and two days later same symptoms. Changed crank shaft sensor this time and still same symptoms. The car has had a full service fee weeks ago including new fuel filter. Also changed coolant temperature sensor and all new coolant. Any ideas? Thanks