Car

I have a 2008 ford focus 1800 automatic. Everytime I take it on a long journey ie 900 kilometres the engine loses power and it says on the dash board Limited speed mode Engine system fault. Also the little engine motif appears in the speedo. When I stop for an hour or so it is ok. I had itn checked at the ford dealer and they said probably lack of oxygen going through mountains but it was ok now. Just driven a long way again and same problem. Engine only done 29000 Kilometers.