'enormous aerials' are (IME) very often of no benefit whatsoever when it comes to freeview; they are commonly
a) more frequency sensitive than they need to be and
b) more directionally sensitive than they need to be.
The second is a hinderance when setting the thing up, and the first means that you can buy an aerial which works very well for one set of stations but is utterly hopeless with another. They are meant to keep all the broadcasts (in one area) within a narrow band of frequencies, but in practice if you look at what your freeview tuner is actually trying to do, it is pretty clear that a frequency sensitive antenna may be doing you more harm than good.
I would suggest that you try an antenna which isn't so frequency or direction sensitive, and that (if you have not done so already) you set it up using a signal strength meter.
Bad signals in the rain often means you have a water problem, specifically getting into the cable itself. Masthead amplifiers are (IME) likely to be affected by water sooner than you might like, both because of the nature of how they work, and how they are often installed.
cheers
