1977 300d charging issue

[nonextant]

Hi,

I have a 1977 300d and am experiencing the following symptoms:

battery holds a charge just fine over the weekend.
when driving, the radio started powering of when using the brakes..
now, when left idling the radio will power down.
when driving, everything has power, but I will occasionally hear a relay click from the driver side under the dash and my instrument lights will get brighter for a brief moment.

it started fine this morning and again after i got to work, but after driving around fort Carson for 15 min, when I went to start it turned over very slowly.

I believe the alternator is not putting out enough juice at idle or if the draw is too much, ie., headlights plus defroster and heater fan. But is putting out enough to recharge the battery during my freeway trip home.

I will check the wire from the alternator to the starter for corrosion tonight but am wondering if this is a common problem and see if anyone else has experienced it.