Not that I know a lot about it, I've never raced on dirt, but I see some numbers you might take a look at.
You have a big percentage of the rear weight on left rear tire. If the stagger is not right, that might cause a problem.
To me it's a fundamental thing, too much rear bite, push, too much front bite, loose.
Whenever my sprint kart pushed I would narrow the front or widen the rear.
Being indoors, assuming there’s no banking in the turns, maybe you need more stagger. Do you have any idea of the radius of the turns? You said you change the stagger, but you didn't say how much.
1/5 mile is 1055 feet, that's a pretty big indoor track. If you knew the radius of the turns, my software might be able to give you a better idea on a stagger number.
Comments, compliments, criticisms and questions always welcome.
If the data does not support the theory, get a new theory.