Lack of power fixed, loose spark plugs

So if any of you work all recently I test drove a 4Runner that has the same v6 as the second gen. I was really blown away by the sheer amount of power and acceleration that had compared to my prerunner.

At the time people just said it was probably a difference of maybe the gearing or the fact that the engine was tuned a little differently... But I wasn't really ever sure about those ideas...

But now I know what was going on.

I changed my spark plugs this morning, slightly past 30k. I'm glad I did, three of the six plugs were loose. I've never seen that before... Anyway so I change the plugs and when I put the new ones in I torqued them hand firm. My torque wrench is 1/2 drive so the plan was to go to town and get an adapter so that way I could actually torque them to 15 foot pounds.

Man!

The truck has such better acceleration now and doesn't make as many strange sounds or do as many weird things as it did before. Vrooooom!

After I got the adapter and got home, I waited about 5 to 7 hours to give the engine time to cool down. I'm not sure exactly how long it was, I kind of lost track but the engine was only slightly warm to the touch so I figured it was good enough. Again I begin the painful process of getting all the shit out of the way so I can access each slot.

Startlingly enough two of them were already somewhat loose again. I made sure that they each torqued to 15 foot lbs. I took her out for a test drive to make sure everything was okay, once again the increase in power and acceleration is night and day.

So any ideas? Should I not use antisieze? I am a bit worried... I don't know if I should check it again after maybe a couple of days, and if in fact some of them do become loose again how on Earth would you fix that?

(Also in case anyone was curious, I attached a photo of one of the old plugs)