Hi Ken, If the original GM injectors are still installed then it's a good place to start looking. The OEM injectors are not resistant to the ethanol used as an additive in our gas now a days. Generally when the OE injectors start to go bad it takes a little time for the coils to start shorting out. Usually about the time the coolant temp gets to closed loop the miss starts.
Did you try to take an ohm reading on the injector coils?
Did your mechanic pull the plugs to eye them up?
Did he take an ohm reading on the plug wires, from one plug thru it's assigned ign coil thru to it's other plug in the paring?
The knock is the dual mass flywheel responding to uneven ( a big miss ) power pulses.
Does the car smell lean, or rich at the exhaust outlets?
Did he put a scan tool on her at idle to look at the fuel numbers, cross counts, O2 voltages?
Tom