Why does my truck still stall after a few miles or days?

Recently replaced fuel pump after finding that there was not enough pressure at the test port (also replaced fuel filter with pump). Drove around the block after repair and truck stalled but then restarted very sluggish. Due to the mileage I noticed the cat had never been replaced so I pulled to front 2 O2 sensors and checked the cat. Part of the inlet of the cat was melted and cause blockage. Removed and replaced cat with new one. Now the truck drives better but will stall without notice. Right before it stalls I hear it begin to start poping (lean out) before it dies. I have checked fuel pressure and intake mainfold for leaks, but now I'm stuck and don't know what to try next or where to start.