Ford Transit dirty turbo

MY 2006 turbo-diesel Ford Transit has done 250,000 km, but it has being giving trouble since 120,000 km. Almost every time I try to push it up a hill on a freeway the yellow coil warning light comes on and it goes into limp-home mode and dies. After I've stopped and turned it off it will reset itself and I can drive on again. It can happen three to four times per day, but could then only do it every day for a week or two, and then it might go for long periods without doing it at all. It only does it if I'm traveling at over 80 km/h. My mechanic reckons it's the turbo that needs to be cleaned. Have you heard of this problem?