I would guess that the Ford heads are aluminium?
My question is are the plugs blowing out because:
A: Have been over torqued?
B: Is the aluminium corroding at the threads, maybe like some kind of electrolis?
Loose plugs I think would be noticeable, by sound and performance.

Just some thoughts!

 

Do not use a helicoil, The little end will break off and ruin the cylinder wall... Give www.blownoutsparkplug.com a call and talk to them.

 

The head was designed with not enough thread area. Ford updated the heads.

I've also read not to use Helicoils ... to do something else. But, I don't know.
I was between a rock and a hard place, and I had to pay Ford to replace the head real quick for me.