I know it's the obvious but have you checked the head screws for tightness? Have you hydro-locked it? Take the head off and look at the shim (gasket). It's all metal (copper?)and may have a bit of metal stuck in it.

I have a .40 and it did the same thing. I took head off, inspected shim, reassembled, tightening alternately, like wheel lug nuts, opposite sides, and it works fine. Checked screws after running and was still tight. You can possibly get more shims where Irvines are sold if yours is damaged.

Anyone else have any ideas?

PT