My two cents…  start by reducing the overall rear width and then proceed to take caster out.  Its been a number of years since I ran TaG but I never tightened the third bearing and/or had the rear bumper tight (this doesn’t mean it’s a bad idea, but for your reference it is not something I ever used on the karts that I owned).

Arrow Kart has one of the best set up manuals of any kart I have come across.

http://www.wrkc.on.ca/wp-content/uploads/2013/01/Arrow_Setup_Guide.pdf

Loosen up side pods, rear bumper, if you have rear torsion bar remove it, remove one seat strut, and go higher on tire pressure. Leave the caster at max.

Where is your seat at from axle to lip?

Where is your front and rear track at?

I respectfully disagree with most of what Freddy recommended. Tightening the pods and bumper add grip and so do struts. A stiffer axle could help as well. 55 is pretty wide in the rear for that kart. I’d narrow 10mm per side. Reducing caster could certainly help the balance as well.

Mojo tires?

I agree with Gary. Narrowing the rear will help the outside tire dig harder. Also your caster might be putting too much weight jacking through the outside tire and overloading it, causing it to break free.

I used to own that very chassis. I bought a new AX-9 at a nice discount  because the X-1 had been in the market for a couple of years. It was better than my Sodikart SR-5 (all-32 chassis but without the narrow waist, had an archaic frame design, and was very stiff), but I struggled to match the pace of the drivers who had a 30mm chassis for the newly-spec D2 tire. I wish I had known that before I bought it.

Your AX-9 is all-32mm kart, and as such is stiff and produces a lot of grip. It was very good for a hard tire like the D1.

The X-1 that replaced the AX-9 is an all 30mm kart, developed because Mojo introduced the D2 tire, which was grippier.

My point in telling you this is to suggest that tightening a kart that already produces a great deal of grip is going to make the kart less flexible–perhaps unflexible–hence the sliding. I’m assuming, of course, that the kart isn’t worn out from years of use.

To me, everything you’ve done to the kart is making a stiff kart even more stiff.

Having said all that, T.J. is a lot smarter than me.