Get you a Chamber Maid.
Your DPMS upper is marked as .223/5.56 on the left side of the mag well, and if you look at the bottom of your barrel in front of the forearm, you should be marked 5.56 1-9. That is what my A-15 is marked. If so, then you should be able to safely shoot .223 as well as 5.56 in it. So, that being the case, you probably are not getting the shavings from using 5.56 in a .223 chamber which will sometimes happen.
Strip it and clean it. That is likely your problem, then try it and see what it does. What ammo are you using, you did not say. That could be an issue also, if it is imported ammo. Try some Federal or other American ammo and see how it likes it. Those AR's have to be kept clean and wet. Check it and get back to us.
After you get it all cleaned, then lube it good. Here is a link with your lube points.