Pre 1994.5 36v Marathon intermittent failure - also worth upgrading?

[Ajsemtb]

I have a 1994 (pre 1994.5) series marathon, bought used for $950 (probably too much now that I'm reading about y'all finding these for free or a couple hundred...) And I've had it for about 3 months. Starting after month 1 it started failing randomly, and it will stay off for several hours, or the next day, or night, then it will work again. I "THINK" I've narrowed it down to the throttle Pot, but still not so sure. Also, I thought it may have been water/weather related as it seemed to fail after the first drive after a rain. And up until this last week, it wouldn't seem to fail in warm dry weather. But after this last week of failing, don't think it matters.

So, here's what I do know: 6 Trojan batteries dated 2015 mostly with 1 or 2 from 2017; there is some swelling present on the tops of a couple of them. All water topped off with distilled (only 1 was low, and it wasn't really low, more of a "top off").

All battery cables removed, terminals and ends cleaned and dielectric put on.

Solenoid tests functional and it always clicks, even when the cart is disabled.

Pack voltage is 38.3 after a charge.

Runs 10-12mph when running, which is why the failure is between the throttle and controller.

The Pot tests from 0-6.3k ohm, so I recognize this is above the 10% range of 5.5k from 5k but that is pedal to the floor, so i should be able to just shorten the linkage, yes?

But what else could it be if it's NOT the Pot?

I can't find a troubleshooter for the Alltrax NPX controller, and I don't know which pins of the controller should have what voltage key on/off, so I haven't troubleshot that for sure yet. So any leads on a troubleshooting guide for the NPX would be appreciated.

And finally, I'm new to carting, but looking at all y'alls carts has gotten me really interested in some upgrades/customizing, so just curious what the lift/tire upgrades cost, what is solid state drive? and is this old of a cart worth the money to upgrade it? Thanks if advance.

[Ajsemtb]

Oh, and it can/does often run for days/week with no problems at all.