The metal used in the earlier RIAs were not to today's standards.
There are several causes for such a problem.
The sear spring (left finger) is the one that keeps pressure on the sear, check that.
The mating surfaces of the sear and hammer may not be correct (as stated by Greg_E).
Parts could be wearing.
When was recoil spring changed? Consider changing that too.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
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Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)