Clintonshane

So I had my engine rebuilt due to bad valves at 186,000 miles. Got re conditioned heads and replaced everything but pistons and cam. The sucker ran tight but good for about 2 1/2 months. Then, misfire. Felt like a wire or plug, so I changed them and found oil IN one plug cylinder. So I immediately thought valve seal since it wasn't smoking. So I installed a no foul until I could find umbrella seals, and low and behold,now that plug is clean as a whistle. No oil on the business end whatsoever but it still misses whenever there is any compression behind it. Particularly when trying to maintain city speeds of 30 to 45 mph. Anyone have any ideas?

It is a 2003 F150XL. 4.2 V6