Intermittent Miss on Cyl 3

I have a 1996 GMC Yukon with a 5.7 Ltr engine (vortex).  I am having an intermittent miss between 20 and 60 MPH.  It is more pronounced in 3rd gear, and less so in O/D.  The code has come up as a misfire on cylinder 3.  I have replaced: Spark pulgs, spark plug wires, dist cap, and rotor, and have put in all new injectors.  Still the code comes up Cyl 3 Misfire.  Can anyone suggest what the problem may be?  Thanks for the help!

Probably a leaking intake manifold gasket.