We had a similar problem with our upstairs TV. Downstairs TV fine; upstairs TV losing signal, etc. Both connected to same aerial, so aerial obviously ok.
We had a guy come out and check the signal with a meter. The signal was poor, so the guy put up another aerial in the loft (cheaper and easier than replacing the coax from the aerial on the roof).
You've poss got a similar problem, bad signal caused by damaged coax or connection .... poss caused by water ingress.
No problems for us now :)