TiVo : strange tivo problems

[dodgem22]

Hi all I was wondering if anyone else has experienced this or has any ideas on what the issue is.?
I had a samsung 1tb tivo for about 6 months it then developed a fault as if the sound was dropping out for a split second every ten minutes or so.
Virgin came round checked all connections cables signals etc and said they were fine and swapped the box for a 1tb tivo cisco all was fine for about 10 days then the box started to crash every couple of days.
Virgin again came back around and did the usual checks and said the box is faulty and swapped it for another 1 tb tivo cisco and now again 10 days or so later this box has started to crash. The engineer insisted when he brought this one its brand new and not a refurb. I thought cisco did not make the boxes anymore and all the new ones were samsung?
Any thoughts or ideas as im very perplexed by this
I did note that on no occasion was the power pack swapped could this be the source of the problem?
I have another Tivo 500 gig samsung and this box has not had these problems.