Hey guys I have the exact same issue.

I installed 757 in FSX:SE and seemed to work fine, untill I engaged the autopilot: 757 became inflyable, start banking left and right continuosly and I got in stall.

So, I reinstalled (repair) and start from cold and dark, but nothing change. Just till I engage A/P my speed fall down, I got stall and a/c begin banking left and right as before.

Have you got an idea about what can cause this strange behaviour and how to solve it?

Thanks so much