Blazer Stalls / will not crank

Today after driving about 10 minutes my 1985 Chevy S10 Blazer stalled and when I went to restart it no sound at all was heard from the starter..... 3
hours later it started right up...... but after 5 minutes of driving it stalled again...... so I jumped the battery and again it started right up...... when I got home I put in a new battery since I knew it was over 5 years old anyway........ I also noticed my charge indicator gauge is just barely out of the red danger zone of low charge........ so is this an alternator problem and why would my car start right up 3 hours later after the first time it stalled if the battery was so low that it couldn't keep the car running.........
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