Just as a point of information, I dropped GA several months ago and I still get very clear periods of zombie traffic. (Although my overall conversion rate is slowly climbing. YMMV)
The biggest problem I have in analyzing these periods is that Google seems to be very careful to turn them on/off at odd periods of the day. Since the granularity of most analytics software is one day, it's hard (without custom software) to split out these periods to look for differences.