1997 Chevy Suburban LOSING POWER UNDER LOAD

This is my second engine in this Suburban, another 454 cu in. The first one blew, cracking the block, and this one has maybe 8,000 miles on it.

We've been scratching our head over this problem for some time. The 454-V8 runs fine while the engine is cold, accellerates like a skitterish puppy. But when the engine heats up to anywhere near normal, it starts missing and bucking under load, no codes. The warmer the engine gets, as in on a longer trip, the worse it runs, but it idles fine under hot or cold conditions. Sometimes pumping the gas pedal helps straighten out the bucking while under load, but nothing makes it right.

We've done the distributer, plugs, wires, changed the fuel pump (ouch$) and all to no avail. Any ideas?