Maybe not your problem, but back in '87 or so I built up an AR-15 carbine. I would jam often enough that I was ready to get rid of it. Checked headspacing and all and didn't matter what ammo I used. Went out shooting one day with BIL and used one of his magazines and it didn't jam! All of the used surplus magazines I bought were worn/warped enough to give me fits.
If all else fails after you've followed these other guys advise on cleaning and the spring, try a different magazine.