Duke is correct. Each transistor is turned off by a negative spike approximately equal to Vcc. If the peak value of the spike is greater than the transistor reverse Vbe rating, repeated applications of the spike will weaken the transistor even if the spike current is limited. One solution is to add a small signal diode (1N914, 1N4148, etc.) in reverse parallel with the each transistor's base-emitter. However, this will alter the timing calculation and increase the oscillation frequency.