[Supra_Freak_7m]

If you are using an oem head gasket with the head machined, that could be part of the problem, as the cam timing will be off due to the slightly shorter distance from the bottom of the head up to the pulleys, and where the cam timing is off, the igntion timing is probably off too. This also increases compression, which can cause detonation issiues? Alot of v8 mechanics LOVE to blame that turbo for everything from the car overheating to the tail lights not working.... Its just ignorance for the most part. as long as your not boosting too high for your current fuel delivery capacity and tune, it SHOULD not be the turbo.

SO, with that said,

Did your head/block mating surface get machined?
If So, How much was removed?
What were your head bolts torqued to?