@ "Any chance a capacitor before the driver would smooth the PSU spikes if there are any? "
Don't think the SwMode PSU would really appreciate driving into a large capacitive load on startup, however once started it would certainly smooth ripples depending on the capacity in question.
Either way this might be a bum steer but the car battery would remove any possible doubts, if you get same readings it means problem is NOT the PSU at all.
Cheers
PS. A pot across the car battery voltage could also supply you with a nice smooth modulation voltage which you can set with your multimeter.