This is a very old unit, so we have little information on it.

My best guess is that this means that there is a DSP overload. This is particularly likely during preset loading.

You may find that it occurs with specific presets.

If you are using digital input, it may also suggest an unstable digital signal. If so, make sure the sync source is appropriate and that you are supplying a good signal (good cables, leads not too long, etc).