Depending on the charger, a regular auto type charger should not be used to run anything on it's own. It's voltage goes up and down based on the load, and you could be seeing spikes as it goes up, and "Brown Outs" as it is asked to supply more current than it is capable of. Every one I have purchased has a specific warning about doing so.. I ignored it the first time, and quickly destroyed all my 12v lighting
in one of my Pop Ups thinking I could run straight, rather than from my battery.
Your cold cathodes actually run on 120v or more, and draw very little current, but it's still an invertor that is actually supplying the bulbs. They are temperamental based on what it going INTO them.