It almost sounds like your top is too lean and bottom too rich. Bottom end masking being lean up top.

It oscillates after clearing it out or just when say idling on the box for a while (basically after no throttle input)?

Other things to check would maybe be your tank and lines for vacuum leaking. You may have another issue making your engine seem like its the culprit. If your orings look good they are most likely just fine (not always the case though lol).

Backplate seal look ok?