Blue? Ecch - we need to talk one of these days - you should be using Super Whites!
If you are losing alternator output, more draw will cause lower voltage. The regulator is supposed to keep output up when the load in incerased, but if you have done any electrical upgrades involving higher drain on the system (brighter lighting, high-powered stereo/amps, and suchlike,) you will start losing what may have been a perfectly good alternator. When you get a replacement, make sure to get a higher-rated unit (at least 100A.) It's better to have more power available than you need, as any unused capacity will be available for later, and it will be there when you need it later. As long as you don't use all of the capacity (leave a little over - I try to leave 10% overage for any electrical generating capacity.)
If you can find a good local shop, I recommend that route. I have a shop that does my rewinds - install and forget. It's nice having a stable alternator putting out a nominal 70A at idle and capable of almost 150A at full output....
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