Pain--fit, saddle, core, or fatigue?

[Owlie]

if you are not stably situated on the saddle, it is easy to rely on your arms/shoulders/etc to hold yourself in a proper position. I had this issue with my old ill-fitting saddle -- it was too narrow for my sit bones and i tended to slide to the side or forward unless i kind of held myself in place with my arms (which led to considerable arm/hand/shoulder fatigue). A new saddle has largely eliminated this problem

I've definitely had that problem with the stock saddle. That led to all kinds of pain. Replacing it helped a fair bit, but now it's back.