I'm running out of suggestions...

Check syslog for anything useful and see if you can post to the mailing list to get the attention of Kalle as it may be interesting to get to the bottom of this, and perhaps get a fix if it is a software bug.

If it's a hardware fault and/or you don't have the time to debug this (which would be understandable) it may be necessary to take the 770 back for a replacement (assuming it's new) or send it away for repair (in which case you may want to debug the problem further as this could be the quickest solution rather than wait for a repair!)