Does anyone have any suggestions?

I have a 2002 GMC Yucon Denali V8 6.0 L with over
325,000 miles. It has a new battery, alternator, serpentine
belt and tune up. My headlights/rear lights flicker and dim
most of the time while driving or stopped in drive,
sometimes lights will flicker while car is on but in park.
Electric windows go up and down slowly. Some of the
interior lights have gone out. Dash lights dim.
Speedometer sometimes reads I'm driving 125 mph when
I'm not. While car is on and or driving, battery gauge
needle fluctuates back and forth from 10 to 16. Check
engine light comes on and stays on sometimes. Car starts
to overheat sometimes. When taking off from a stop car
will always sputter when accelerating. When slowing down
and or turning the car will die sometimes. I don't  know if
this is all tied together or if I have numerous problems. It
would be greatly appreciated If you could give me any
suggestions. Thank you.