99 Gmc Sierra Running Rough - Help!

[soundsgoodllc]

I have a 99 Sierra with the 5.3 and around 205,000 miles. It's been a great truck so far, but recently its been giving me some weird trouble. Sometimes it does fine, and other times, it just won't run at all. I'll be going down the road when all the sudden, I notice I have no power when I press the gas. It will sputter and backfire and just choke and die if I try to run over about 1500 RPM. If I turn the engine off and let it sit a few minutes, it's ok to drive again until the next time it hits - could be a few minutes or a few weeks... The only code that comes up when I check it is the Rear O2 sensor, but my truck has been throwing that code constantly for at least 5 years, this problem is only a couple months old. The first time it did this, I took it to the shop and they said it was the fuel pump and replaced that and the fuel filter, but it obviously wasn't the problem.

 

Any ideas?