Ok, so I'm posting this for my 16 yr old bro in law. Saturday, they purchased a 1991 2dr, 4.0, 4x4. He went to drive it home, died 1 mile down the rd. They jumped it, it does again. Had it towed home. Yesterday they changed out alternator and worked great. Then noticed the starter was hanging up some after start. Installed new one. Then did fuel filter for good measure since prior owner just installed fuel pump. Also has after market intake, throttle body spacer, new plugs and wires. Still factory exhaust, which made no sense to me.
Well, this morning he got on the road to school, got to bout 55mph, and motor died. He turned around to take it back home and died 2 more times. It kept starting fine, but don't know the issue. Any suggestions? I think I included all info. Thanks guys

 

Any check engine lights? By chance do the gages go dead?

You'll go broke with the shotgun approach for parts replacement. Start by checking grounds and hot wire connections are clean and tight.
Search member Cruiser 54, he has lots of great tips.

 

check the crank sensor. mine was doing that. i'd pull over and it would crank right back up-----untill one day!!!!

 

On my truck the engine kept dying. For that it was my alternator. If you've already changed the alternator check that the connection to the battery is working properly and it is being charged correctly.

 

I was having the same problem, my ignition coil was cracked, and it was grounding out on the block. Replaced it, haven't had a problem since.

 

I appreciate all your input. It kinda helps that my other brother in law is a pretty good mechanic. They got it narrowed down to the fuel pump last night. He pulled it out, and could tell it wasn't new, like the seller claimed. Got it changed out this morning, and he drove it to school with no issue. I will keep all of these things in mind, and thanks again.