Use 'act up' for gadgets or machines causing frustration due to malfunctions. If your printer starts acting up, try restarting it before calling support.
Pro Tip 2/3
Mischievous Behavior
'Act up' often describes children or animals misbehaving in a playful but troublesome way. Whenever there's a full moon, the kids in my class tend to act up more.
Pro Tip 3/3
Non-serious Context
'Act up' isn't generally used for serious or criminal behaviors, only for minor disruptions. My phone's acting up, it keeps dropping calls today.