why is my mk 2 mondeo revving wildly?

help i'm going to go crazy lol! i'm currently not the proud owner of a mk 2 w Reg 1.8 mondeo lx. after numerous problems which have been sorted this one is the most worrying as nobody seems to know the problem. occasionally at random times the car revs wildly when i try to accelerate. it pulls off really slowly and does not pick up speed as it should especially if i'm going up a slight slope. when i do try to accelerate the revs will keep going up to around 3000 rpm and dropping again until i reach the desired speed. it is so frustrating especially when your pulling off from a junction and the car decides to pull out like a snail and rev like wild horse. (and no its not because i'm a woman driver lol!) please help