I agree about the firing pin spring. If the firing pin tip doesn't look smoothly rounded it might be best to get a new one. If you haven't changed the recoil spring for a while I'd also get a new one of those in addition to the firing pin spring. Total cost is probably less than $20 for Wolff springs and maybe $15 for a firing pin.
These new parts might not solve the issue, but it's a simple and cheap first step that eliminates a bunch of possible problems.
