Weird Electrical Issue

[drizler]

2000 GC 6cyl Laredo auto 100K miles. Recently it began starting sluggishly when very cold. 6 Years on the OEM battery, not bad at all but I could see the headligts dimming somewhat so went and got a new Interstate HD batt big enough to start diesels.
Car still seems somewhat hesitant when its -15 F but starts right up. I still notice that about every minute or so the lights flicker as does the blower as it sounds like something "shifts". Its instantanious and barely noticeable but still there. Volts stay steady at just above 14 V. What would kick on and off about every minute during driving that could cause this? I was thinking that the voltage might need replacing and alternator starting to give up the ghost. Any more educated ideas??? 100K always seemed like a long time for an altso a rebuild isnt much out of the question right? Anything else that acts like this?