I also have a T/C Encore 209x50. I have not had your experience at all. In fact quite the opposite.
My rifle shoots 1.5 m.o.a. groups @ 100 yds all day using the following set-up:
3-50/50gr. pyro pellets
1-220gr. " Dead Center" sabot bullet (made by Precision Rifle Bullet)
1-Winchester 209 shotshell primer (actually thinking about switching to a .410 primer)

I seat the charge firmly, add the primer and fire!
I am using the original breech plug and the rifle is 3 years old!

My point is to try different bullet / powder combinations to see what works best in your rifle. You may be surprised at the results.