2005 Blazer, 6 cyclinder, 4x4

Crazy instrument pod/cluster!!! Speedometer sticks sometimes, registers wrong speed sometimes, works sometimes. Had it checked out, but no code or electical problem found. Also, sometimes the headlights flash on their own or if window is being drawn up by the electic switch on drivers side door, battery gauge will show negative flow while window is operating. Any help out there???

Go over all wiring and fuses and plug In's.
Both grounds and positive feeds.
Start at battery cables and ends.