Had exactly the same problems. Have a Start 25. Could never find a signal, and when I did it was lost easily. I could manually set the time, but it was lost every time device was turned off (constant 12:00). I updated last night---the Nav & Quickfix updates solved everthing---signal comes end quick and clock keeps time all the time. I have an even older TT gps (Via X?) and it hasn't been updated, but except for the clock it seems to work fine. I wasn't allowed to update the second gps. It seems you can only have one registration per PC. However I noticed after I plugged in the clock is working now! Here's a temp solution (it was for me anyway) if you can't update:
(1) You must RESET your gps----EVERYTIME! You must hear the Tomtom drum beat when the logo comes on. When you turn on and just the Tomtom logo appears you WON'T get a signal. After the "I Agree" stuff appears and you accept it will take 4-5 minutes before you get a signal. Before then you can manually set the clock, or SYNC once you get a signal. This is a pain in the ass method, but it worked in getting a signal.
(2) You said you updated the QF. When I updated last night, before I could do anything else like update my maps I had to make TWO updates----a) QuickFix and b) Nav.?---the site requires it. I don't know how you did one without the other if you truly updated QF. You do them both, or none at all. They are BOTH pre checked for updates. Maybe your gps is too old. My first one was bought in 2008 and was the same model as yours. I used it everyday and only got about 2 years out of it.