1997 zr2 with the 4.3 vortec. 285K miles, 100K on the engine. I am getting misfires on cylinders 1-4. Just had a full tune up on it including plugs, wires, cap, rotor, ignition coil, fuel&air filters, pcv valve, and had a shop do a fuel injection cleaning service on it. I even pulled the cap and rotor to see if anything was wrong and they were fine. It shudders and stuff at low rpm then goes away as I pick up the speed, and is fine on the highway. What could it be??