Use the light kit inside, and also turn off any florescent lights. Bright sun can also give strange readings.
Eratic readings can come from several things ,a very weak or stiff arrow that is not going straight, or if you shoot crooked thru it.
Weak battry's also cause grief, so a plug in is better.
The consistant area should be about 4" directly above the traps.
There are tutorials on line for setting them up,lighting etc, just google it.
Many often find they vary up to 20 fps when shooting. A chrono is a great teaching aid for a clean consistant release.