I had the same problem with a J&T CAR-15 upper on a DPMS lower. When the unit was new it fired a few rounds, then jammed, fired a few rounds, then jammed. In my case (no pun intended) it was due to the mags. I was using no-name 20 round mags. I think they were Pro-Mag AFAIK. Anyway, the next time at the range I used seven-round Armalite mags and it's functioned well ever since. The 20 round no-name mags worked in my Eagle Arma/Armalite M-15A2 so I thought they would work in the J&T/DPMS. Oh well, live and learn.
Speaking of J&T, I finally got to meet T of J&T at the Pomona gun show today. What a nice, cordial lady she was.
While at the Pomona gun show I was able to find some used 20 round USGI mags. From what I understand, these should work well in the J&T/DPMS CAR-15 and in Bushmasters also.
Good luck with your BM, thequickad.
Later,
Rich