Elercterical issue

I need some help here with electric issues, been having this for while now. Can't seem to figure it out.

Here are my issues:

1. My cluster gauge backlight will get brighter and dim.
2. My Voltage meter on my obdII will fluctuated from like 14v to 12.56v when idle and driving.
3. My AC blower will turn up and down while driving and idle.
4. My dual battery gauge goes up and down with the voltage meter reading.


The weird thing is it only happens when it is hot outside or when engine is fully warmed up.

I have check my ground wires to the battery and even sandpaper the main terminal.

should i resurface the positive terminal with sandpaper? when i mean sandpaper i have one of these.

please help.

Truck info.

2nd Gen Tacoma 2007 4x4 with 130k on it.

The battery ( Die hard Platinum 1100cc) and Alternator is Mechman 240amp. Both are fairly new.

Have dual battery system installed which the secondary battery is stock tacoma battery.

I have 2 led bars, 1 Arb air compressor , winch, back camera all hooked up to a secondary fuse panel.