I have a Hitachi P50S601 that comes up with a full gray screen. The blue light blinks twice then a pattern of three that keeps repeating. I have tried to remove both buffer boards to see if the tv would stay on at the top or bottom and it didn't.
Your TV actually turns on which is pretty interesting given your PDP errors. First thing you should do is get hold of the service manual for your model tv (or one very similar to it).
There are links to manuals (pdf) all through this hitachi section.
Then you should get a multimeter and test your SDRs (dont just pull them out). That will tell you if they are any good. They maybe bad but to the naked eye look perfectly fine. Next you have to check your power supply for defective componnents. This is a visual check for swollen caps. Even a slightly raised cap can be a sign of defect but know that a bad cap (like a bad sdr chip) can be bad but look perfectly fine.