I had the same problem with my car. Found out that when I adjusted the valves I tightened them to tight and the intake valve was hung open. Ran the car on 7 cylinders for 1300 miles. With 7 cylinders it dynoed at 357rwhp uncorrected. Try a compression test and maybe that could be your problem.
its the inside of the intake. Thanks Yellow I'll check that out. I haven't had the heads off my car. When I bought it it had the GT-40Y on them. This problem just recently occurred though. So I'm at a bit of a loss.
i had this happen when my ajustable fuel pressure regulator took a poop. somehow it was dumping fuel in through the vacuum line to the intake. Not saying this is your problem, but maybe check it out.