Happened to me with the same model & FW....Hmmm. Panasonic said to send it in to a center.

Guess the only other alternative is to leave it on during the day & turn it off at night to get TVguide updates.

I noticed lots of models, even those with no hd or that can add new firmware, get this U99 code.

Have you tried calling a Panasonic service center to ask them about it?
Here are a couple:
WESTGATE TV- STEREO
841 W HAMILTON AVE CAMPBELL CA 95008 (408) 379-7670
TV UNLIMITED
641 GRAPE AVE SUNNYVALE CA 94087 (408) 733-6913