Hyundai 2.5 Turbo Diesel Motor Year 2006

Hello, I have an Hyundai H-1 year 2006, 2.5 Turbo diesel engine and my problem is as follows; The cold game is very good, the engine is in less than 2 seconds, but when it reaches the working temperature I take as 15 seconds of start, at height over the 4000 meters accelerates like an animal and is very good but the slowdown is erratic it varies between 500 and 700 rpm. And the smoke of the exhaust is black, I turn it off and I turn on again and that is the strange, does not light Check Engine, all the maintenance per day and original fittings, the distribution belt reviewed 2 times and I no longer know what it can be. Other similar cars do not have this problem and reach 5000 meters high. Thank you