We have had Freeview UHF since TiVoHD first came (3 years ago?) and reception has been perfect. Suddenly in mid October 2012 we got reception problems. It was most evenings, and occasionally during the day. It affected all three muxes, but Mediaworks was the worst.
I found out that my workmate who lives 540m away (as crow flies) had exactly the same problem - perfect reception for years then suddenly mid October got breakup in the evenings.
I have been able to mostly eliminate the issue at our house by removing all the VHF elements from our combination areial. The last few days we have seen more breakup on the TVNZ mux (only on shows recorded in the evening).
I remembered I had a VHF attenuator which I gave to my workmate, and that seems to have solved his reception issues. He has a UHF medium gain antenna (straight elements, with corner reflector).
I have found out that my neighbour across the road has also had issues, and had Ascot TV come and fix the problem by fitting a filter (I haven't inspected it yet). She has a 'Gizmo' compact VHF/UHF antenna.
I believe the issue is something on the UHF band which has only been broadcasting (or emitting) since October. I can't think of anything that has changed locally, other than perhaps the Barrington Mall expansion. Could be refrigeration equipment.
The time of day is interesting, kind of implies it could be HAM radio.
We also have an analog TV recorder (TiVo Series 1). Before I removed the VHF elements from the aerial I recorded shows on the analog recorder at the same time as shows on the Freeview UHF recorder. Unfortunately I couldn't see any obvious interference on the analog shows, when there was breakup on the digital shows.
I have a complaint open with Radio Spectrum Management, and I am keen to hear from anyone else with a similar story to us.