I've experienced that problem several times on my old ( WWI-WWII )military pistols. In my situations, it is always in the sear/sear spring/hammer notch area. When the slide slams home, it jars that area enough to release the hammer. So if the messed up geometry is just so, the gun will keep going full auto
I'm betting that's your problem, and the previous owner got a little aggressive when polishing the sear/hammer area ??
