If you just bought it 3 weeks ago, have you:
Changed the oil and filter? (5w-40 Full Synthetic is highly recommended)
Changed the fuel filters (there's 2)?
Used a fuel additive? (Our Trucks were built to use the pre-2007 Diesel, the new low Sulfur stuff has stuff removed that our trucks Need to run properly, a quality additive is a near-must with these engines)
If you haven't done any of the above, I highly recommend doing them first, unless you have proof they were recently done, and done right. These trucks are very sensitive to the quality of the oil and fuel going into them. If there's any problems with them, the injectors will let you know.
Once you have done the above, and if it still does it, then you move on to the next step in injector problems, which is trying Rev-X in your oil. It has been shown to reduce stiction by removing varnishing inside the oil paths in the injectors that gets caused by dirty/old non-synthetic oil, that cause them to not work properly.
You're welcome to go and straight up replace the injector, but that's about $250 (Per injector) in parts alone, if you do all the work yourself. In contrast, Rev-X can be had for ~$50, it treats all injectors, and it has been confirmed to alleviate injector problems by dozens of members of this board.
Only then, after you have tried all of the above, would I recommend actually changing the injector. The worst the above treatment will do is clean out your fuel and oil systems, and make them the best they can be, even if it doesn't fix the injector.
