It is reported that laser transversal and longitudinal mode structure as well as other factors such as pumping rate disturbance, causes instability of the output of a cw Nd:YAG laser passively Q-switched with Cr4+:YAG. Effectual methods of improving stability are given and a stability better than 5% is gotten. It is suggested that the thermal effect of Cr4+:YAG and damage to optical coating should be taken into account when increasing the output power of the Q-switched cw laser.