Assuming that your table, arm and cartridge are set up correctly:
100 pounds of cement stores a lot of energy. With the bolt heads sitting on carpet, the carpet decouples the cement stand from the floor and this energy has no where to drain. Your turntable, having no suspension, becomes part of this energy block. The result is the turntable getting upset by the large amount of energy it cannot dissipate.
Suggest you experiment with a light weight but rigid stand with spikes or tip toes to pierce through the carpet. Then try using tip toes or sorbothane feet under the turntable to see which works best.