NPR Smoking and running ruff

My 99 diesel NPR has about 200,000 miles on it and has been well maintained. Several weeks ago it started smoking (black) on and off. Oddly, it would run fine for five miles (city driving) and then a little power loss and it would begin to idle ruff and smoke sporatically. It would smoke constantly at 52 mph. Above that it would be sporatic. Any suggestions?

first check filters, fuel and

first check filters, fuel and air. does the truck sit alot? the fuel could be full of algie, use a good algiside, tank clean from a international heavy truck dealer, if no to all is this truck intercooled? check boots clamps and the cooler for leaks. a small hole will cause the simtoms you stated. do you have a low vaccum light on? if so i would check the plumbing before the turbo for leaks.

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