Hi, several months ago, I had a Timney trigger and a stronger firing pin spring installed on said rifle, that I seldom use, but I've found a quite hard bolt lift since, when I shoot; to be more clear, most of the bolt lift is smooth, then I feel a sudden, sharp resistance at the end, that need an extra strain. If I cycle the action without shooting, everything seems ok. I suspect the issue to be caused by the cocking system and possibly by the stronger spring and/or the Timney trigger. What are your thoughts?

P.S. My reloads are OK, no high pressure signs, easy extraction, no shiny marks on case heads.

Can't see how the Trigger would impact it. I would vote for the stronger spring and maybe an issue at the end of the cocking ramp.

Try the following:
#1. Remove the bolt , pull the trigger and press down the sear to its deepest point.

The sear must dive free completely down. Sometimes one has to round the corners/frontside of the sear if the fit in the slot of the rifle is too tight.

#2.
Looking from the side the sear has a front bevel edge that maybe too steep (or short), so that the bolt can be lifted by force , but not easy (after pulling the trigger).
Sometimes you have to correct this by filing/sanding to the proper angle.

Don´t alter the engagement point of the sear.

I've reinstalled the original, weaker firing pin spring and put a little grease; now the bolt lift is noticeably smoother. Thanks!