Same exact thing happened to me...you might need a new head...when I pulled my head off I found that the spark plug was hitting the top of the piston and that some of the valves got banged up.
I have a 2g head and a 2g 7bolt block if you need it.
99 GST - Some stuff
14.083 @ 97.37 (10psi-ish ATCO 3/13/07)
"She said its JUST a car...I said your JUST a girl"