Yes, I'd tend to agree. I'd still ask TT if I were you. Might be some kind of known W8 issue.given it happened with one OS only and that it is duplicated across two machines (current one running at about 38 deg C), seems to me that a physical slot etc is more unlikely than a card issue. As you say the driver is pretty old.
Quite a few options:Any thoughts on a multi tuner DVB-S replacement?