I bought a 646 for IDPA about 15 years ago that had all the same symptoms as the OP.
First thing I did was get out the gauges and measure everything on the gun. The cylinder had excessive headspace AND the front gap was already at max. I called CS, gave the measurements, and stated I wanted to use the gun for IDPA. CS sent shipping label and said to include a detailed letter of measurements and what I wanted done.
S&W pulled the barrel, set headspace and BC gap to specs, and returned N/C.
Function was then reliable.
Then I did my thing with competition springs and hand-lapped C&S firing pin to set up for IDPA, and installed red FO front sight.
Ran flawlessly; still does whenever I get it out.

Due to rule changes in IDPA, I seldom shoot it in competition now, preferring to use optional 130 pf .38 with speedloaders in a Model 66 to save stress on aging hands.

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Science plus Art