I would certainly be looking at cleaning the firing pin channel. If oil gets in there, that will gum up a firing pin and cause light strikes. Plus, while you've got the firing pin out, you can check the springs to make sure they are ok as well.
I agree ^^^^
If after that the issue persist, you might want to get a new set of springs.
Recoil spring, Main spring (hammer spring) and firing pin spring. that will make you almost start from scratch, because so much the previous owner could have done.
Luckily for you, most owners don't go file things down (like I sometime do
), but only replace OEM parts with aftermarket stuff without really knowing of the outcomes.
Many aftermarket parts should really be install by a gunsmith, not because owners are not capable of the install, but because the outcome might not be what was expected, or just doesn't fit, like the 11lb recoil spring sold for CZ 75 by CZcustome or CGW, but in reality doesn't not fit the CZ 75.
