Starting problems

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Got a used 96 Trooper 2 months ago. Running pretty good then one day couldn't start the engine. First I changed the battery. Seemed like that was the problem, then after a week, same problem. I turn the key, it started to crank, then it died.The dashboard went dim, then no power. Waited 20 minutes, crank the key,power came back but it won't rev the engine. Can hear fuel pump working. Might be alternator or starter. Has anyone gone through this?