This might sound like a stupid question but is the blowback coming from the primer hole or from around the breech plug threads? Did you tighten the plug that same way as before? Have you tried different primers? I see you said that you tried Winchester primers but was that to troubleshoot the blowback or grouping?

You should be getting great results from your encore. My best combination is 200 grain T/C Shockwaves, 130 grains of Pyrodex pellets and Winchester primers. I also have the stronger lug spring and 1x hinge pin. No trigger job yet.