suburban stalls when warm

I have a 98 chevy suburban 5.1 liter auto, 4 wheel drive 144k mile. After the suburban is warm 15-20 miles it starts to run poorly and stalls the check engine lite goes on and I had autozone put there code reader on it said oxygen sensor. I replaced 2 of the three in the exahust system it did not help. after talking to a friend he said it could be the fuel pump. I had the fuel pump replaced 14 months ago. this problem has been going on for about 6 months and is getting worse with the warm weather. if it is the fuel pump why would it only occur wher the car is warm. am I missing something or should i replace the third sensor the one after the catilytic convertor or is there another issue. Symtoms are does not respond to gas peddle well and will stall , it starts up and runs for a short time 1-2 min and stalls again. eventually it will not start untill it is cool 4-8 hours. then it runs fine for 15-20 miles and thern same thing. I can hear the fuel pump humming when it does act up but cannot tell if it is the problem.
please help and I will make a donation if your advice helps me save some money Bobanddeblakeside.