Jzzz3

Ok, so im driving the Z and all of a sudden the car starts sputtering, and Im getting very little power. I take the car to the shop, they scan and it reads that its a cylinder 3 misfire. We check the cylinder and see that ignition coil is totally fried. We replace it, drives good for 20 minutes, misfires again. I drive the car back, cylinder 3 coil is fried again, we take out the spark plug and see thats its totally bad. Its NGK iridims put in maybe 3k miles ago. The plug could literally be spun freely. After the plug and coil is replaced, it misfires again after 40 minutes of driving. I finally take it back, and we see that all 5 other plugs were starting to get loose and go bad. We just replaced all and the car drive perfect for the 30 minute drive I took it on. Does anyone know as to why that would happen to the plugs? The grounds looks fine.