optra magnum diesel turbo underboost

i own optra magnum diesel 2008 model,mileage run 75000km.since 50000km my car pick up got down.if the car crossed 130km/hr and made it to run for 15 min,then turbo charger gets under boost and some check light is glowing,if i continue driving after glowing check light car not crossing 100km/hr,by observing with tech2 service persons are clearing the problem and the same was repeating if the car crossed 130km/hr for 15 mins. service persons cant find the solution till now