I bought a matched pair of 75s in nickel plate back in 1975, and one had similar problem. I was working part time at The Flintlock In Hobby City in Anaheim, and the owner, Ray taylor, and I workd on that gun for over a year without any success.
I finally solved the problem by selling the guns to my Sergeant Major for his two sons and he had someone make him a whole new one out of stainless steel.
Looked funny, but worked very well.
Sevral subsequent guns (all blued) have not had any problem.