On the Varian INOVA 500 you may notice that the tuning display is unstable while you are tuning the probe. On the Bruker spectrometers, this may show up as ripples or waves on the "wobble" tuning curve. This phenomenon is due to Q modulation in the probe as a result of spinning the NMR tube. The tuning of the probe is dependant on the orientation of the NMR tube within the probe. This problem is much worse for cheap NMR tubes or NMR tubes which have been damaged or lack perfect radial symmetry. The effect can also be seen for solids samples spinning in MAS probes.
To avoid the problem, simply stop spinning the sample.