1973 mark

hello everyone, i have a ford galaxy 2003 zetec. you go out in the car and all is fine, you then go out for a second time and you can't get over 40mph and it goes into limp mode. you pull over and turn the car off, restart and the problem goes and the car is good as new. the problem doesn't happen all the time. the last two months it's been good as gold but then it happened restarted and working fine. can any one help. we changed the turbo sensors(what the garage said to do) at a cost but didn't get rid of the problem. please help if you can, many thanks, mark. p.s the revs still go up high but more speed and it to limp mode.