The rollers are controlled by the 12V solenoid on the valve block. There is a 12v constant supply when the rpm is over the set amount.
Two wires run from the solenoid to the circuit board,these steer the solenoid by cutting the power and stopping the rollers when the rpm drops below a set amount.
The rpm is counted by the sensor picking up the spokes of the pulley,the gap between the end of the sensor and a spoke should be about the width of a pound coin. To check sensor function turn the ignition to on,have someone rotate the pulley past the sensor and observe the light on the circuit board illuminate when a spoke passes it,then blink off.
Check continuity in the wires running back to the control box.
You say that after an hour the problem starts,could be the solenoid but they are generally very reliable.
Failing that,remove the circuit board for a look at the back of it. Look for burnt spots or even slight discolouration . Post up a picture of the circuit board,make sure you take a picture of what order the wires go.