1998 xj 4.0l. Will start strong sometimes, but other times just click and all power goes out. Battery is new and holding power. Starter is new and tested good after issue started. If I disconnect negative battery cable it starts great. Has newish terminals and they've been cleaned since my new battery was only last month. I saw a thread on here with an identical problem but I didn't see any conclusion. Can someone help me please? I'm leaning towards replacing some grounds, but wanna know if this could be the computer / security malfunctioning? Thank you

 

That symptom is usually related to bad connections, especially bad or corroded ground connections. I did a small video about them. Make sure to check them all and clean them all up. Its free so start there.

I had the same symptoms on my 96 last winter. It ended up being a worn out negative battery clamp, and some anti-corrosion products I had used around the battery posts (because the clamp wasn't tight enough - because it was worn out - the product had worked its way between the clamp and post). But any bad connection - especially poor ground connections, could cause the problem you describe.

 

Found the issue right under my nose. I don't why I didn't look at this closer. Felt like an idiot when I found it. the steel brackets the pinch the wires to the terminals looked good from the outside, but when i decided to take it apart the underside was all rust and it was dripping into and covering the wire. That's why it always seemed to be worse on rainy days. glad it was simple cleaning. I'm switching them out for brass this weekend. I still plan on going through the rest ground wires though.

 

Thanks for the follow up!