Your alternator is getting lazy. It triggers by RPMs, and now at idle the magnetic field that it generates is appearing to be growing weaker with the age of the part. You should look at having it tested. It may just be borderline.
What is your battery charge level? Does it fluctuate?
The charging circuit appears to have an issue, but you won't know how bad until you test it.
It might help to check your battery connections as well as the alternator power connections to be sure nothing is loose.
'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
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