Hi,
Check that the voltage from the alternator/voltage regulator is stable and not fluctuating.
To do this, put the transmission in park (AT) or neutral (MT), firmly apply the parking brake and start the engine.
Connect a voltmeter across the battery terminals and measure the voltage there.
It should read between 13.8V -14.5V DC.
have someone rev the engine (up to 2500rpm) for 30 seconds and make sure that the voltage remains within the limits above during this time
If it fluctuates outside these limits there's a problem with the voltage regulator or the alternator.
Be safety aware when working in the engine bay when the engine is running!