how can i fix the over revving problem on a 1999 ford focuus?

i am having exactly the same problem of over revving every time i am stopping at trafic lights,juctions or give ways.its a 1.8 ztec petrol engine,1999 model.it seems that the garages cant or dont want to tell me what is causing it and just keep charging loads.please help as it is costing me a bomb. thanks