overheat

Hi all, just wondered if anyone has any idea what could be going wrong with me mate;s dads Discovery. I know its a diesel and i know its not a td5 like mine, i think its an earlier one, but he has a problem that keeps cominng up and i thought you lot would have some good ideas. The problem is the vehicle periodically overheats with power loss and runs ruff..while in this mode. Sometimes he can make journeys with no problems and then like coming to work today he overheated again. He tellls me that he checked all water and oil etc and al seems ok Any idea's would be appereciated Mark.