Trigger "bouncing" out of sear on '73

[August]

Has anybody experienced the hammer following the bolt extension down after cycling the gun (at match speeds)?  If so, what did the problem turn out to be?

Background:  Gun has worked flawlessly for over two years.  Suddenly (after a complete disassembly and cleaning) it started to drop the hammer prematurely when running fast.  It seems like it's bouncing up out of the sear engagement, or the trigger is bouncing out of the sear, but that's just a "sense" I have about what's going on.

Trouble shooting so far:  trigger sits firmly in sear when hammer is cocked.  No amount of pressure on the hammer will cause the hammer to fall, suggesting the sear is GTG.  Replaced the original, highly modified, mainspring with a new C&I spring (unmodified).

The new mainspring seems to have reduced the number of unexpected hammer falls, but it has not eliminated them completely.

What should I check next?  I hope that's enough information to get you thinking.  I don't know what else to check (other than the trigger spring).

Is there some aspect to the two piece trigger that might cause such a problem?

Thank you for any and all help.