I also had this problem and it got increasingly worse. Eventually worked out that it was the tuner in
the Humax that was slowly failing. Have a read at this thread :-
Sorry it's not more cheerful news
So what are you going to do about it? Is it still under warranty?
I'm wondering whether I have a related problem with my HDR-T2 which is just under 18 months old, and thus still under warranty. At first I thought it was temperature related and only occurred when the house was unoccupied for a few days, and cold - but it has since occurred at other times, too.
I've had very few completely failed recordings - although it did, for some reason, fail to record University Challenge last Monday - and told me that the recording was "less than 30 seconds long" when I tried to play it. The main problem is that it occasionally behaves as if it has a very weak signal - sometimes with complete picture and sound breakup, so as to be unwatchable. This can happen both on recordings and when watching live.
When watching live, if I switch to the TV's own tuner using the same aerial feed via the T2, the picture is perfectly ok - suggesting a problem with the T2's tuner. But it's intermittent, and I couldn't guarantee to be able to demonstrate it. If I send it back to Humax, they may well find nothing wrong with it!
FWIW, I'm on firmware version 1.02.28 as a result of an OTA update applied on 26th May 2012.