Skipping 96 Altima

My 96 altima has started skipping mainly at idle when warm. It only has 215,000 miles so imagine that! Its still a good car though doesn't burn any oil between changes. The skip is not noticable when cold, I've changed the plug, checked the wires, installed a new o2 sensor, and checked and cleaned the EGR valve. When I pull the plug wires while running the wire clsest to the distributor (#4?) makes very little differance in the way the motor runs, but it is not a dead skip. I removed the injector it looks good, do they cause occasional problems? The car seems fine at highway speed but the fuel mileage seems a little off. The check engine light showed a code po400 and a po308 I think. I had them cleared at Auto Zone it has stayed off. I've been searching the forum and have seen the intake gasket failure info but I cannot hear any vacuum leaks. I'm leaning towards the injector being bad any advice appreciated very much. Thanks :banghead:

The first diagnostic step after your visual inspection would be a compression test to prove that the engine can run smooth. Is the valve cover leaking any oil into the sparkplug tube(s)? You could switch #3 & 4 injectors with each other and see if the missfire moves to cyl #3. The sparkplug or wire could be bad. Check the wire with an Ohm meter even if the wires are new. Use something like WD-40 useing the red plastic straw to spray around the intake gasket. My guess is that there is a vacuum leak that can't be heard.