Along with the above...
check for clogs in the fuel rails. I was getting a code for missfire and after changing a lot of stuff out still had it. Took a screw driver and tapped on the fuel rails which apparently loosened it up...no problem since. Although
I was still glad I changed the plugs...the PO had put in some cheap crappy plugs.
Also I used to have vapor lock problems if my fuel cap wasn't on right. This occurs when the vehicle warms up (about 20 minutes) on warm days. If you open your cap and hear a hiss sound as the pressure releases this could be the problem.