ian_pop,
My advice was based on my reading of your post, I thought you had had a working DNTV card that appeared to stop then tried 2 more of them, neither of which worked.
Its a sad fact that some systems behave differently than others, and some cards are more snsitive than others to signal problems. In my system I use one one DN PCIe Dual Hybrid S1 and two avarmedia A777s, have also used a DN Quattro in place of the dual hybrid. In it I can not get the A777's to scan channels but the DN tuners work fine, so what I do is use the manual hack system for 4 tuners but make sure the last channel scan is done by the card that works, in my case the DN card, that way the channel scan that is saved is a working one and all 4 tuners pick up all channels without a problem.
I would add my cables are all quad shiels and connections are F type and have a new (well two year old now) digital antenna and my antenna bloke says the signal is ok.
The manual reg hack process I use is this one:
http://www.hauppauge.co.uk/board/sho...ght=tuners+mce
Don't know if the tuner extender app can be used in the same way, would have thought so, but as I could never get it to work for me I don't know.
Give it a go and see what happens, I would also look at doing as dgaust suggests and have your antenna etc either replaced or at least checked.
Craig.