Panasonic TU-CT20 h/w, s/w, or reception problem?

Do I have a hardware or software/firmware problem with my Panasonic TU-CT20 DTV converter, or do I have a reception problem?
Symptoms are:
If subtitles are switched on, after some hours (4 commonly) they start to slow down and eventually freeze altogether. If nothing further is done, then a few hours later (3 commonly) the picture itself freezes. The sound performs normally at all times. The only way to unfreeze the set - no controls are by then effective - is to interupt the DC power to the unit. No pixelation is seen on the picture at any time. And the 'signal quality' on 'setup' is fine - all green. In all other respects, if I don't use the subtitles, the unit performs correctly. After discussing the problem with the Panasonic factory helpline, I've just exchanged the unit: the same symptoms are apparent.
Can anybody throw any light on this?

Sounds like a software (firmware) problem, doesn't it.

However, are you "cooking" your receiver? They don't like to be pizzafied. Keep it on an open, cool shelf.

One other possibility is external interference. This can enter via the mains power cable or, more often, through the aerial cable. This is why you should always connect the aerial with a continuous length of double-shielded coaxial cable (eg. Satellite TV cable). Any wall plates/sockets should be fully shielded and the aerial and cable should be kept away from mains wiring and other sources of inteference (eg. traffic).

Fman

Thanks Fman - good points.

You could be right with regard to heat. The unit is in the open, but is operating on its side. There's no space on the shelves in the cabinet. So it's supported on the side of the cabinet. I think I'll try opening the unit - maybe just separating the two halves of its case by a centimeter or so to allow more ventilation. (If I'm careful I should be able to do that without visible damage, to preserve the guarantee!)

But interference shouldn't be a problem - quality coax, and connections used throughout, and we're well away from traffic and other sources of interference.

It'd be interesting to hear if anybody else has experienced the same symptoms - if it's a software or firmware problem I shouldn't be alone...(?)