The gun is apparently a 9x19. I use Wilson 15# springs in my 9mm 1911 Pro and it functions properly with just about any ammo. It does rather sound like the gun is not quite fully in battery for the first round, but without seeing the gun it's just a guess. Try loading just five rounds in the magazine and see if that cures the problem. If it does, it would reinforce the idea that the gun is not quite in battery - possible due to the recently replaced slide stop and new magazines. If those are the troublemakers I imagine that with a little use the gun will settle down.

PS - I spent many hours in a 20C that my Dad flew. (5850Q) Wish they had made ANR headsets back then.