car problems

i have a problem with the car which i bought at ford dealership,after my 30000 km service the started giving me signal that says Service engine now. i would take it back they would put on diagnostic machine and say its fine. since then i have been a regular at the garage with the same problem. the other time they booked the car in and told me they have changed wiring. the car was fine until taking it back for 45000 km the problem of service engine now started again. since then am struggling with the car when that sign come up the car looses power and cant change down. i bought a brand new car because i did not want to get stuck on the road as a female driver. i proposed they change the car for me they told me my problem is minor. please step in to help with this problem as i am now feddup and need a new car to serve the purpose of having a new car. ford dealership the glen