Printer HP 8720 firmware error 69666

[pastim]

When my 8720 starts up if looks OK initially, then flashes a blue screen, but so fast the error code cannot be read, and then shows a black screen with 'printer error' and asks me to turn off and turn on again.  

Most times this does not work. When it has worked I have managed to look at the log, and it reports a firmware error 69666.  The printer then performs perfectly well, aligning heads, cleaning heads all OK.  Until the next time it is started from cold.

The firmware is up to date. I have done a normal reset and restart to no effect.

Some of the ink was low, so I have put new ones in.

I believe there may be a way of doing a complete reset to original firmware and start again, but I have not yet found out how to do it.

WHat does error 69666 mean, and is there anything I can do about it.

[pastim]

Further to that error message, the Printer Hardware Information from HP Smart includes log entries like:

Error Code: C4EB8447

And also C4EB0400

and also

93 74900 1 TapNo=10;Name=DIAGNOSTIC PAGE;Desc=Diagnostic

92 69666 1 3303769159

90 74899 2 FROM_NVM (and also 74900 and 69556)

[pastim]

The problem is still happening, albeit not every time I turn the printer on.  Is there any list of what such error codes mean?