Normal 8-month babies do not become stiff sometimes.
If a baby gets stiff briefly off and on, remember how long the stiffness
lasts, whether the baby is awake & alert during the stiffness, and how the baby behaves after the stiffness. One should speak to the doctor after the stiffness. If stiffness is associated with a significant illness, or is more than a few seconds, call 911 for ambulance.