Some connection is heating up momentarily and you are losing contact. Even if you checked your battery terminals you probably have not physically chased those wire and you should also tighten those battery cables. Simply giving them a wiggle will not do it.
This is obviously a very dangerous situation and you need to take a very careful look at it yourself even with limited skills.
Chase every wire off the batteries. Take pictures and post them here.
Once you find any connection where the wire goes, tighten that connector. Make a mental note of anything that you can tighten even a little bit or worse, a full turn or more.
Twelve volts just has very little pressure/push and will not push through a weak connection.
Follow the negative leads off your batteries to wherever they go. When and if you find them on a frame bolt etc., remove those and clean everything clean and shiny. Use sandpaper, wire brush, dremel tool with drum sander, or whatever works for you. Get them shiny and clean.
If you have any battery cut off switches, you may need to dismount them to tighten wires that may be connected on the back.
Be safe working on batteries. Always wear safety goggles.
Myron & Deborah
08 HR Endeavor 40 SKQ
400 Isl, 400 HP/// Now 450 HP
2016 Ford F150 Limited V6 with Ready Brake