1999 Chevrolet Silverado fuel issue I think

I have a 1999 Chevy 1500 with a 4.3 I'm having a
really uncommon issue with. I'm a mechanic and
I've asked several and no one knows what the
issue is. The truck does this thing where after
driving for so long ( seems to be after about 20-30
minutes of driving) if you go to lay into it say
getting on the highway or passing another car it
starts to spit and sputter and won't gain speed
when floored, but it just slowly gets worse the
farther you drive. It gets to the point you can only
go to about 2000 rpms other wise it will not gain
speed it just spits and sputters. If you shut the
truck off and let it cool down it does fine but then
after a while of driving the issue comes back. I put
it on a scanner and it was only throwing all O2
sensors. They were both replaced and same issue
still the cats were replace before I bought the
truck. I honestly I have no clue what the issue is.
Has anyone had this issue?