Fishermanferg..... unsure what the question is.
However, what you describe is perfectly normal, if you switch your lights on the energy has to come from somewhere ... and very basically it's the engine. (ie the alternator is 'loaded up' & this in turn saps power / rpm from the engine.)
Your car is no different, switch on the lights + heated rear window & likely the same result.
If your CM 400 Alternator is the same output as the 400F (156 W) you're running it close to its limit .. eg (55+ 4x5)w headlamps + 5w tail + 20w alternator field + 25w ignition = 125 watts
Brake light 21w and / or indicators 42w will soon exceed the alternator output - but they are only transient loads & will be absorbed (temporarily) by the battery. Point is, if your alternator is rated 156W then don't add in too many extra loads.
Lobo.