Here are some questions I'd like to ask to begin fine-tuning the discussion:
1. Are the keywords providing the zombie search traffic the same as those that provide converting search traffic?
2. During the time periods when zombie traffic occurs, are the rankings changed in any way?
3. What is the on-site behavior of this traffic? Do they show more pages per visit? Fewer?
4. Do the sombie periods happen at the same time of day or day of the week?
5. Are there any notable geographic differences?
6. Is the total traffic high enough for these changes to be statistically significant?
1. How much variation is there from high to low traffic levels?
2. Is the traffic capped on a daily basis? Weekly?
3. Does weekend traffic cap the same as week day traffic?
4. Any clue HOW Google is accomplishing this effect?
5. Do you think it's intentional on Google's part, or possibly an unintended artifact?
I think those affected need to study the bejeezus out of their data. Fire up those spreadsheets!