I went from the stock one to a 270a mechman unit in 2010. idr the mileage it was swapped at since I didn't make note of that stuff at that time. It lasted 4 years and then the bearing on the rear of the alternator literally exploded and made the rotor rub onto the stator and get so hot it was starting to burn itself up. I swapped it out with a 250a mechman alt on 10/25/14 at 134,272 miles and all has been good until the one time I had to gun it to get onto the thruway since some d-bag wouldn't let me on and I noticed that when I got my engine speed above 6000, the alt would stop charging. Turns out that I was really over-spinning the alt and if I kept doing it, I would have killed it. lol. So lesson learned, with a 1.75" diameter pulley on the alternator, do not go above 4400RPM on the dash. If you do, the alt will work until it gets too fast but its not good on the alternator. The stock alt can handle higher speeds since the pulley is larger.
With your voltage dropping down like that, you could fix it with a smaller pulley on the alternator so that you can increase the speed of the alt and get more output at idle. Downside to doing that is you need to watch how high you rev your engine.
If you want to make sure everything is alright, get your alt load tested and check the voltage ripple to make sure it isn't anything crazy and that the output level is good. If it is, you can just ignore the voltage dropping some or get a smaller pulley and smaller belt to rev up the alternator speed and fix the problem.