Sorry for a second reply, but before ordering any parts, I would consider the following:
1. Are you sure you don't have a bad box of ammo with bad (hard) primers? I once had a box of PMC ammo, and every fourth round simply would not fire, due to bad primers. Other boxes of the ammo fired without problem. So, get a different box, preferably a different brand, and see if they fire OK.
2. Take the bolt out, and soak it with Gunscrubber, clean with brush, and then soak it in Hoppes, clean with a brush, and then soak it with WD-40, clean with a brush, to make sure that the firing pin channels and holes in the bolt are not rusted or clogged up a bit.
3. Take the firing pin and very very gently polish it with 400 and then 600 grit emory cloth, just to slick it up a bit.
It is possible that one of these foregoing may solve your problem.
Also, remember that eventually, firing pins do wear out in the ordinary course and need to be replaced
Hope this helps.
Mannyrock.