Hey Sonnytex,
TimC Has made me aware of your problem,, So hopefully I can help add to your confusion..
I fly an OS 70 surpass II on my Twist, I have not encountered that problem at all with the Original stock engine mounts,
question...... I know this was asked earlier but I saw no answer, Did you shave the mounts on the inside to accommodate the motor?? I know its just a hair tight but I just wedged mine on there, it slid nicely into place by a little hand pressure.
I have even buried the plane into the ground a few times, as a matter of fact this past weekend was my 4th major rebuild of the bird, Love this plane,, Anyway I use 4-40 bolts with lock nuts on there, I snug them up, but not tight, just as you said you do, after reading through the thread, I would say maybe it has something to do with the things the others are telling you about the unbalanced or spinners or vibrations.
What size prop are you spinning?? I use 12-8 for speed, 13-6 / 14-4 for the better 3D stuff and I like the 14-4 the best so far, I don't have a prop spinner on it any more, as it cracked on my first rebuild and its a little less nose heavy without it.
and There is nothing like this Combo,, Twist & OS 70, I too love the way she rockets into orbit,, talk about your unlimited vertical, it flys awesome as well, my flyin buddies love when I roll out & within 2 feet just go vertical to no end.
I hope this was somewhat helpful to you.