As always:
Verify fuel pressure, and assure the fuel filter is OK.
Make sure all the "tune-up" parts/procedures are OK. Cap, rotor, plug wires, spark plugs, PCV, EGR, EVAP, heated air intake systems all working properly. Initial spark timing OK, electronic spark advance working properly.
Assure the plug wires are routed as GM intends--including all the plug wire looms.
Replace the O2 sensor if it's got more than ~50K miles on it.
Look at the spray pattern of the fuel injectors. Should be a nice, even cone of fuel spray from each injector. At low speed, the injectors fire sequentially, but at higher speed this is so hard to see that they look like they're both firing all the time.
Connect a scan tool, verify EVERY sensor and computer output. Look at fuel trims. Assure it's got an RPM signal that varies with engine rpm, and never "drops out".
