this is usually caused (form some ive seen come in) by too much timing in the map somewhere causing it to switch to the knock maps (richer).
Best bet is to get some decent knock detection (gizzmo K-mon, knock box etc) and tune the car, switch it off and do another run if you find the AF changes copy your fuel map into the knock map by pressing KNOCK COPY button and this will ensure both maps are the same. Turn the car off and the restart it and do another run, and repeat. if the A/F is constant after making the two maps the same then you know the issue is your timing map (causing it to switch). Remmeber to richen the knock map up after your done fixing the issues in your timing map..
So far ive had no issue with shifting A/f values.
The NISTUNE stuff is extremely good value for money and im yet to find any real limitations in 99% of applications.