Given the age of your 39-2, I'd replace the hammer spring just as a matter of course, as it's inexpensive and easy to do, to eliminate that as a possible cause.
If it's still the original spring, what you're experiencing could be symptomatic of a weak hammer spring, where an otherwise "normal" amount of residue in the firing pin channel now offers up enough resistance to slow the firing pin travel and cause the misfire, where the same would not happen if the hammer spring was still up to the task.
Wolff carries the spring, their
website indicates that same spring was used in all generations (#27520 20# spring):
GROUP A Pistols - Long
For Use in:
39*, 439, 539, 639, 3904, 3906, 3944, 3946, 9mm
59*, 459, 559, 659, 5903, 5904 and 5906, 9mm
5923, 5924, 5926, 5943, 5944, 5946, 5967, 9mm
909, 910, 915, 9mm
645, 745, 4506, 4526, 4536**, 4546, .45 ACP
4566, 4567, 4576, 4586, 4596, .45 ACP
1006, 1026, 1066**, 1076, 1086, 10mm
410, 412, .40 S&W
4003,4004, 4006,4026,4043,4044,4046***, .40 S&W
*Early 39/ 59 pistols had a 22 Lb. factory spring.
** Some pistols may have short hammer springs
***Some 4046's have a 16 Lb. factory hammer spring.
•Reduced Power...: 16, 17 & 18 Lb.
•Factory Standard.: 20 Lb.
•Extra Power........: 22 & 23 Lb.
