2003 Tahoe, 5.3 liter vortec flex fuel. Engine

2003 Tahoe, 5.3 liter vortec flex fuel. Engine replaced at 60k miles due to low end knock (typical late model GM). Scanner shows mutiple misfires on all 8 cylinders and erratic timing. Rough idle. Everything replaced, knock sensors, ckp sensor, even the computer. Everything else tested, checks out fine, fuel pressure, oil pressure, compression all good, even the Chevy house can't figure it out. Any suggestions?