chevy truck stalls

I have a 93 chevy 4x4 with 117,000 miles. On my way to work it just quit-- died, It did that for about 3 days always starting right back up. Then it
stopped restarting and a mechanic told me it was the fuel pump, so I skeptically changed it, That didn't work-- no real surprise. It was runing for 15 to 20 minutes, now its down to 5 minutes of running and not restarting. I have to wait 15 to 45 minutes before it will start. What would cause this? I do have spark, but there is no gas being injected into the carb. any ideas?
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