Cause for an intermittent misfire on cyclinder no. 4?

I have a 1998 GMC suburban 1500 with 5.7 Vortex engine. My "service engine soon" light came on and the P304 code indicated a misfire on cylinder no. 4 cylinder. I have changed distributor cap, and the plugs and wires twice. The light continues to come on every 50-100 miles and go off for 50-100 miles and comes back on. When it is on, the truck has a slight miss, otherwise it runs OK. What could cause this intermittent miss? Thanks, Ken, Houston