So my sisters jeep (98 xj) is having a strange problem. Sometimes it will start just fine, but other times, as soon as the key is turned to the start position, ALL power immediately goes away and the battery is instantly dead.

The battery and alternator were load tested and are good. They are both less than a year old. On the way to the auto store to have them tested, the volt guage on the dash went all the way to 19 volts for a couple of minutes.

I replaced the negative terminal connector because it seemed loose.

Also, she told me that sometimes after it fails to start, power will come back after a few minutes, but then the issue reoccurs.

It starts and runs when jump started, but the gas pedal must be floored (which cuts fuel in half) and then it won't idle by itself for a few minutes, like the iac was completely closed.

I am leaning towards the battery cables, but I'm not sure.

Other info:
In Nevada, so little to no rust or corrosion
Also, I briefly looked in the fuse box and I didn't see an asd fuse. Is this possible?

 

I'd say you're right, it's prob the bat cables or terminals. The ASD is a relay in the fusebox in the engine compartment.