Turning on heavy loads should force the alternator to output mode. This should increase the system voltage (and lamp intensity). What is your power-off battery voltage?

Your regulator may be switching at a relatively low frequency. Everyone has a different tolerance to refresh rates on a computer screen, some people (me included) are irritated by the 60 Hz refresh on a computer screen or from fluorescent fixtures. I need to run 80 Hz or better on my screens to avoid the flicker effect.