Renault Laguna renault laguna revving by itself

I have Renault laguna 2.0 16v 2003. It has an on going problem whereby after travelling about 8 to 10 miles if I stop and start the journey again, the car starts ok but after few minutes the electrical fault light comes on and the engine starts to rev betwwen 2000 and 4000 revs. and the accelator has no control over the engine speed. The car has been in the garage and they have replaced throttle body, spark plugs and the injector E.C.U. After the ecu was replaced car was ok for couple of months and now the problem have again surfaced and is getting worse. Initially problem was occuring one a month or so but now it is occuring regularly whenever and I make start stop journeys. The distance of the journey I thionk is not important but whenever the car is stopped and restarted within half an or so the problem occurs . If the car is left standing for few hours and then started it will take longer for the problem to reoccur.