1997 F150 4.2L 5 speed.check engine light

I get the code P0306 (which is misfire on #6 cyl) the engine seems to run fine, but when I bring the idle up to 1800 -2000 RPMs the engine does seem to be missing a little. I'm thinking it's in the timing. On a trip I can drive 200 miles with no check engine light, but when I stop for gas and get back on the road with'in 10 miles the light is flashing again (flashes for about a minute). Driving a shorter distance the check engine light will also come on after a short stop. If the light is on or off I still get 18-20 MPG. I have new wires, plugs, fuel filter, air cleaner, #6 Fuel injector is about a year old. I haven't checked the fuel pressure regulator, but it's the next on the list... 210,000 miles 1997 4.2L V-6 stick shift.
Any Ideas
Thanks

I'd look at the coil. Sounds like an ignition miss. Never hurts to re-inspect the plug and make sure there was no problem with the install/faulty part. Anything fuel-related before the injector should affect all cylinders, or random cylinders. The fact that it's only cylinder six says it's a problem with the cylinder or the ignition to to the cylinder. The fact that it's not a constant miss accompanied with noise would point towards ignition over a base engine problem.

the problem was the spark plugs (Champion heat range - 14), installed autolite plugs APP105 (heat range - 5). other than the light flashing I had no problem, it was like that for 2 years. bought it new, it now has 213000 miles runs great.:)