When the frequencies of oscillators are observed for long periods, occasional frequency jumps can be seen.  In precision oscillators, the magnitudes of the jumps are typically in the 10-11 to 10-9 range. The jumps can be larger in general purpose units. The jumps occur many times a day in some oscillators, and much less than once a day in others. The frequency excursions can be positive or negative. The causes (and cures) are not well understood.
	The causes are believed to include the resonator - nearby spurious resonances, stress relief, changes in surface and electrode irregularities; and noisy active and passive circuit components, such as the bistable “burst noise” observed in solid state devices and resistors (which look similar to some of the features in the upper curve above).  The effect can depend on resonator drive level.  In some units, frequency jumps can be produced at certain drive levels (but not below or above, as can be seen in the Koyama reference).  Aging affects the incidence. Well aged units show a lower incidence of jumps than new units (or newly turned on units).  Minimizing surface and electrode imperfections can contribute to lowering the incidence of jumps (e.g., according to hearsay, unetched or lightly etched crystals exhibit more jumps than deeply etched crystals; and plating and frequency adjustment in two steps result in more jumps than "one-shot" plating).    Environmental effects can also produce jumps.  Magnetic field, pressure, temperature, and power transients can produce sudden frequency excursions, as can shock and vibration.  It is not unusual, for example, to experience shock and vibration levels of >0.01g in buildings as trucks pass by, heavy equipment is moved, boxes are dropped, etc. [Note that, for example, 0.02g x 10-9/g = 2 x 10-11.]
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	K. K. Tuladhar & G. Jenni, “Frequency Jumps on BVA & Other Precision Quartz Crystal Resonators and Burst-Noise on Overtone Mode High-Frequency Quartz Crystal Resonators,” Proc. 1996 IEEE Int’l Frequency Control Symp., pp. 339-342, 1996, IEEE Cat. No. 96CH35935.
	M. Koyama, “An Experimental Study of Long Term Aging of Quartz Oscillators,” Proc. 1995 IEEE Int’l Frequency Control Symp., pp. 620-622, 1995, IEEE Cat. No. 95CH35752.
	G. E. Moulton, “Burst Noise and 1/F Noise in Quartz Crystals and Oscillators,” Proc. 42nd Ann. Symp. on Frequency Control, pp. 389-396, 1988.
	M. J. Buckingham, Noise in Electronic Devices and Systems, Chapter 7, “Burst Noise,” Ellis Horwood Ltd., Halsted Press: a div. of John Wiley & Sons, New York, 1983.