power problem

on a 1996 Chevy 1/2 ton driving down the road and totally lost all battery power like radio and interior lights then all the power for a second then comes back on. Then a month later does the same thing again. All battery cables are clean and tight the body , engine grounds have been cleaned and tight. Checked firewall connections seem to be tight (wiggle test). Under hood fuse box and battery connection clean and tight. What could I be missing.

Did the engine quit running or was it just the eletrical acessorys that stopped working?

did both