Vortec timing

I had the 5.7l motor in myTahoe rebuilt, now after installing it, it seems sluggish and misses at low rpm under load, and occasionally at high rpm. I have been told to set the distributor to TDC, and the PCM computer will take care of the rest. Well I have set TDC twice with the same problem. Cap, rotor were replaced , new plugs set to .060 as specified on the underhood decal. The only thing not replaced was the wires, which were no problem prior to engine rebuild, and there are no engine codes Any ideas?