1998 Chevy Tahoe Poor high load performance.

Let me preface this by saying that this problem has existed since I got the car at 85,000 miles.

This is a 5.7, generally runs perfect. Good mileage, 18-19 highway, idles smooth, starts easy, no codes. When pushing it moderately hard like a long hill or hard acceleration on the highway, it starts dropping cylinders after 10 to 15 seconds. If I let up on the gas and slow up they cut back in over about 15 seconds, or so, and it smooths out. It feels like its running out of gas. Over the past year it has new ignition everything, fuel pump and filter, cats and exhaust, O2 sensors. Has good O2 waveforms. Shows no misfires on Matco scanner. Cannot see anything happening on the scanner when it happens. This gets dangerous going up a hill with a semi on my bumper! Any ideas?