83 240D - lights flicker a bit at idle
 

Just that. Fan belt is tight. Alternator is 2 years old. Battery is about the same age. Charging voltage varies between about 13.8 and 14.5 volts.

Should I be concerned or keep a spare voltage regulator in the car? (Being a diesel, dead alt is not a tragedy since motor will run w/o electrical power.)

I would check all earth points main one is engine to body .And including the battery conections .Clean them all with emery paper also the point on the body .The cars are painted with all earth points covered in paint and only the thread to make contact .A little grease on the earthing points will give a good return . And a quick check on regulator brush box .Remember , you radio code can get lost if you remove battery leads .