The fan is thermal controlled. when cold, the fan should be in a disabled state; As temperature rises, a threshold is reached and fans start to oscillate, slowly, and then picks up.
Does your fan ever pick up speed or it just stands there?
Does the amp get into Thermal shutdown because the fan is not operating normally? Amp may go into Protect mode, stops normal operation, and fan should be oscillating like crazy! There may be an issue with the fan drive circuit.