Have you switched you timing chain tensioner to the new one? If not, it could be that. Saw it happen to a friend of mine. He was running a 350z and I tried flashing my lights and calling when I saw the sparks exiting his exhaust. Sadly his motor sit out a few miles down the road. Was due to his tensioner failing.

 

 

Yes, I have a new style tensioner. I replaced it over this winter when I took the top end apart to inspect everything, fix minor leaks, and do a cyl leakdown test.