Peugeot 206 idle problems when warm

when idling once warm the rpms drop and then pick up. I was told when i brought it by the last owner that the EGR valve needs replacing as it had idle problems and they had just put a blanking plate in which solved it. But I've decided to replace to replace the egr valve which didn't solve the problems. Then after talking to my garage who said the ecu might need a reboot so i took it to them to do as well as an new cam belt as i didn't know when it was last done. But after a week of driving it (680 miles) the idle has come back. Also I've notices the turbo has a oil leak could this a cause of the problem as oil is getting on to any sensors?