Focus titanium limp mode

Does anybody know I reason or a solution to my problem.. I have a Ford Focus titanium 2.0 tdci 2008 it's done 120,000miles. When I first start the car when it's been stood a while and I give it some throttle it goes into limp mode and doesn't come out unless I switch engine off and on again but only when it's a first start or it's cold. But Evan sometime not often but still happeneds so it's worrying, when it's warm and I give it throttle on a open road it goes into limp mode and engine management light comes on but only for like a day and it goes off again I have no idea what it could be?? Any help would be much appreciated thank you.