93 Suburban 5.7 TBI No power at WOT, and has low timing advance at WOT according to scan tool.?

I have no power at WOT and NO codes, will not accelerate even one mile an hour when I floor it. spark advance on scan tool reads 16 degrees at WOT. when I back off the accelerator easy, then I can see the advance is rising and it starts slowly to accelerate. If I back off enough for it to allow a up-shift to 4th gear it gets about 30-35 degrees advance and is accelerating even more. On normal driving it seems to work fine. it's only approximately past 50% throttle it seems to act up. I have tested and changed almost anything I can think of by now and still no change at all. New fuel pump, new MAP sensor, TPS has been tested several times on scan tool to double check. test run on free exhaust to eliminate a clogged cat. anyone have any idea or have experience form the same problem? the truck is at about 190k miles. I have had it for about 6 months and the problems has been there the hole time I've had it. Previous owner was an old man that probably didn't know the problem was there. to be honest I miss the good old days when vehicles used points and carburetors, and could be fixed in 2 minutes by the help of a Swiss army knife by the side of the road.