My PSU was flakey for a long time.
It would sometimes boot and identify itself with a different number in the settings box (normal was 9B7 for mine, but sometimes 77F for example).
It is one of the original units. I decided to have a look and discovered some bulging capacitors in the power supply which I replaced and it's been perfect ever since.
My intent here is not to encourage you to go and try and fix the unit yourself.

Get it serviced and if it is the PSU, it might be a relatively inexpensive repair as in my case.

This just happened with my FF800 as well and from a little research it seems pretty common with units of this age (I bought one shortly after release at the end of 2004).  Synthax quoted me a total estimate of $200 to swap out the PSU - does that sound about right?  Seems a little high but then again that includes labor so it's probably more than fair...

Hopefully RME are sourcing a more reliable solution for current products - I've never had a PSU go bad in any of my other gear, some of which is quite old.  I look forward to upgrading to the UFX - the FF800 has been one of the best & most reliable pieces of gear I've ever owned!