Moody Tahoe

I have 1999 Tahoe with a 5.7 and 136k miles. Well taken care of. I have run into a problem lately that happens on occasion. Some days, the truck runs like it did when it was new (smooth, plenty of power and drives great). On other days, the truck lacks power, more effort in braking, and has a skip at idle. I have had new fuel pump, spark plugs, wires, fuel filter, throttle body clean, egr valve, tps sensor replaced just recently. None of these seem to fix the problem. Anyone run into this before?

Thanks

Pat