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.