To act up = misbehave (for people); not work properly (for machines).
"The babysitter was so frustrated since the children acted up all evening." "I'll have to take my car to the shop, as it's been acting up lately."
Ugh, my computer is freezing up again. It always acts up when the battery is low. Let me quickly restart it.
Carlitos, if you don't stop acting up we won't get ice cream later on!
Create a sentence below using act up