I've got a P225 and was its first owner. For the purposes of this thread, it's almost identical to a P6. From what's been described, so far, I suspect the hammer spring has weakened over time and round count.
I shot it until it began having light strikes. The hammer spring (also known as, MAINSPRING) became weak enough to cause light strikes. Replacing hammer springs until I obtained a desired trigger pull cured the problem immediately. I first went heavier (~26 lbs), but this trigger pull was very heavy, probably in the 15+ lb range. Actual pull was unknown as my trigger pull gauge only measures up to 12 lbs. I went down to the lowest weight spring of 16 lbs, which has been working fine for several hundreds of rounds, so far. When the 16 lb spring becomes too weak, I have the next heavier spring already in my tool-box ready-to-be-installed.
Changing the hammer spring is a bit difficult, but I was able to do it with some basic tools, which includes a vise, pliers, mallet/hammer. The roll pin which is on the end of the hammer strut and keeps the hammer spring on the hammer strut can be reused, if necessary, but anything to help keep the hammer spring partially compressed on the hammer strut can help in the reassembly.
If you're handy with tools, you should be able to figure out a method which works for you in installing the hammer string on the strut and reassembling the rest with only average effort.
I did later clean out the slide and firing pin area by disassembling the slide and replaced the pins and firing pin spring while I was in there.
Here's one source to do some hammer spring research:
https://www.gunsprings.com/SIG-SAUER (SIGARMS)/P-220/cID1/mID4/dID6
There are other online retailers from which to also buy these springs or even call SIG directly.