Help 72 blazer stalls

My 72 blazer stalls (only live 6 miles from work). The first time was a hot day and after driving 5 miles she started to lacked power. I pulled over and she died, tried to start her back up but she died again. After sitting for about 20 minutes tried again and she started right up and was able to get her home (temp gauge not working, know I need to replace sending unit). So I replaced thermostat thinking she overheated and had a faulty thermostat. Second time was 2 blocks for home had to stop for a red light and she stalled when braking. Tried to start her up and wouldn't turn over so had to tow her home, even tried to jump start her. Today tried to start her and took a long time to turn over, she usually starts with no problem. Any help in troubleshooting this problem would be appreciated. She has a 350 4BL Holley carb with 3 spd automatic trans. Oil and trans fluid is clean. Tune up and new fuel pump installed about 4 months ago (done by previous owner).

Basics here, do you have spark when it won't stqrt?

Not sure what you mean by spark. When trying to start her engine tries to turns but won't completely turn over.

Spark to the plugs when it it turns over will will not fire up.

I'm pretty sure she does since she will all start up when cold.