Stamina for GMAT

Stamina, have you been doing the tests under official testing conditions? (30m each for two essays, 10m break, 75m quant, 10m break, 75m verbal - all done in a row without stopping / pausing) Have you been taking them at the same time of day? Have you generally not been doing the essays and then did them on the test on which your score dropped? Or did you take that test really late or really early, or did you take it the day after you take another practice test? Anything that might've caused you to be more mentally fatigued than normal?