Q: 2001 silverado LS 1500 4.8 vortech, Power loss Under load, gradual exeleration weak,have to let off & floor it to

Little power pulling a load, when going 30,40,etc, if I gradually press on gas, sometimes it does nothing, as if I had never moved the pedal. 90% of the time it runs great, just no low end initial power, and almost no power under load, pulled my boat up ramp, pedal to the floor, barely made it! I've changed the TPS, & harness,FPR,IACV,map,cam&o2 sensors, fuel pump and filter, spark plugs and wires, transmission fluid and filter, catalytic converter's, checked vacuum leaks,fuel pressure, back pressure, have taken it to Chevy tech, no motor issues, possibly torque converter, Took it to transmission tech, he said he didn't think it was the transmission but could possibly be the torque converter, i'm stumped, could it be the torque converter it's not making any noise it shifts fine and it runs fine driving it normally, just has no power under load, and it doesn't gain speed gradually Like it should, have to let off and stomp then kicks in an goes, next step, trq vrtr, your input?

My car has 150000 miles.
My car has an automatic transmission.