"Sometimes a low voltage light comes on"
The voltmeter is your friend, Get a good one and plug it in..
Ideally you need to measure the 120vac being delivered to the A/C. in practice this is not a simple job,, But you can measure it on any non-inverter powered outlet,
Either a good digital multimeter (Which does other thigns) or a Kill-a-watt is fine here.
Home is where I park it!