my daewoo lanos 1.4 revs are at 2-300 rpm in idle!!?

when my car is in idle the revs can reach up to 300rpm. i was told it was the idle control valve which i had replaced that didnt work so i bought another and that one doesnt work either. its still revving in idle. does anyone have any idea what else it may be. both the mechanic and RAC said it was idle control when it is clearly not. i have got an orange management check light constantly on the dashboard which i assume is from this fault. my only solution at the moment is to disconnect the battery so it reboots itself then it work fine for a few hours. i never had any probs until i had some work done for mot like new cam belt and some new leads...please help