Permanent magnets inside electric motor create stress
There is a reason why DC motors have a convoluted shape to their rotor magnetics: -

Picture from here.
In the picture above, the light grey rotor magnetic material presents to the stator an almost perfect &quot;circle&quot; of magnetic material and thus the &quot;static&quot; effects of the permanent magnet stator are equalized in virtually any rotor angular position. This means it can freely rotate with very little cogging torque. Maybe you need to read up on cogging torque?

the only thing I can use for my rotor is some steel screws

If you are using screw heads, the static magnetic forces are concentrated where the screw heads congregate. To overcome this either make your rotor without magnetic material (a commonly done thing) or try and increase the surface area of each rotor pole a lot more.
Here is another way that cogging torque is reduced. It uses a skewed or twisted rotor: -

The twisted rotor will inevitably have less of a detent/cogging problem.