Checking the spine chart, and not knowing what grain of head you are shooting, it seems your shafts are well within the spine requirements vs. draw-weight/length. Therefore, if spine induced problems can be ruled out, I would go for the following. In addition, I am assuming that the term " twisting" may be " fishtailing" that you are seeing.

1. The intermittent change of impact tends to lead me to believe that your problem may be due to fatigue induced torque. Although a 4-lb increase in draw-weight might not seem a large increase, it can be for some shooters until they adapt (wrist and arm strength). Determine if you are torqueing the bow or gripping the bow when releasing.

2. Move your rest slight left and right to see if doing so eliminates problem, or starts to eliminate problem. If doing so does, do not jump to conclusion that the problem is caused by improper " centering" of rest. The problem may still be torque/grip related.

3. What size of feathers/vanes are you shooting. If less than 5" , the 4-lb increase might now require you shoot 5" .

4. If the rest you are using is the one that has metal pressure blades that apply pressure to the sides of the shaft, the cause might be that the rest blades are applying improper or unequal pressure to the sides of the shaft.

Personally, I would never use any (compound) rest that causes " Berger-Button" type pressure to be applied to the sides of a center-shooting shaft, especially if you cannot adjust the pressure. Pressure buttons or any pressure adjusting device is meant to be used to control the " archers paradox" (arrow bending around riser) when shooting off or against the riser, especially when finger-shooting.