No matter what..... the first thing to do is replace that #5 plug! It may be slightly cracked resulting in an intermittent misfire. Cheap to replace even if the problem is fuel related which yours does seem to be. Make sure the plug is seating fully onto the taper or the compression leak may cause misfire. I suggest engineers blue on the taper to make sure it seats properly, Maybe run a tap down if the taper does not contact the head.

If you had an air leak you would probably get a code , Fords throw this code p0171 174 quite easily although not so readily on our Aeros for some reason.

Swap plug wires between 4& 5 just to see if the code follows.

May well be the injector....... If you go in that deep I would put in a new one; but check the signal with a noid light first. Do you have EGR ? if so then this could also cause the lean misfire problem.