99 gp gt Repeated problem

99 gp gt. 3.8
Cylinder 6 spark plug keeps getting hairline cracks in the insulator part. Causing a bad misfire. Compression good. Coil good. Plug wire good.
Should i assume injecter?

What brand and heat range plugs are you using? Sure you are not over torquing the plugs and are you using the metal heat shields?

In addition to Bob's questions, are you losing coolant, and if so, do you notice any discoloration (orangish if you're still running Dex-Cool, or yellow/greenish if you switched it over to non-Dex-Cool) on the plugs?

No signs of coolant loss. Im using ac delco rapidfires 14. Yep using heat sheild. Id say no to the overtightening question
.
I switched injector today so we'll see what happens.

Your problem is most likely the Delco Rapidfire 14 plugs which can be used but not really a good replacement plug for the OE double platinums. I recall there was a major manufacturing defect concerning them. Here is a pic of one and you can clearly see a crack with carbon tracking. They were exchanged for other brand spark plugs and no further issues.

http://i5.photobucket.com/albums/y186/lizzywiz/acdelco_crack.jpg

I agree again Bob. The ceramic insulator failure is the reason Delco is starting to phase out Rapid-fire plugs. It's also the reason I would never use them.