Typically this symptom is caused by latency, dropped packets or interference on your Wi-Fi network. We recommend an 802.11n router, but realize that's not always possible. Try changing the Wi-Fi channel on your access point and disabling the 5Ghz band. These are common causes of interference in areas with multiple access points in close proximity.
If these steps do not work, we recommend temporarily connecting your AppleTV and computer via ethernet cables to eliminate the wireless network as the root cause of this issue. If you can't use ethernet, try decreasing the distance between your computer, router and AppleTV (avoiding doors and walls when possible).
If you try these without result, it's possible that your computer is having trouble keeping up with the encoding side of things. Try adjusting the mirroring quality to a lower setting in AirParrot's mirroring preferences.