Your motor insulation has a negative temperature coefficient. This means that as the motor temperature rises, like in normal operations, that the insulation resistance to current flow reduces. If insulation temperature goes down, like after a shutdown, the resistance goes up. This change can be dramatic. For every 10 degrees C rise in temperature the resistance is cut in half. For every 10 degrees C drop in temperature the resistance is doubled. Be careful that you are not trying to trend insulation resistance without performing a temperature correction of the readings. Performing a polarization index test during a significant temperature change will provide additional data for evaluation.