Is it normal for my 8-month-old become stiff sometimes?

Yes. Brief stiffening episodes are common. Babies resisting car seats, fighting the change of diapers, or expressing frustration or pain oftentimes triggers stiffening behavior. However, prolonged stiff spells could signify seizures or continually stiffened extremities could be an early symptom of cerebral palsy. So, if your infant demonstrates atypical stiffness, discuss this with your pediatrician.
No. 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.