You have done all the right checks to eliminate your internal wiring and posting your line stats is useful. Your DS attenuation of 63 indicates you are a very long distance from the telephone exchange. Your routers are clearly struggling to maintain a signal but the Speedtouch 330 is known to handle long lines fairly well so that is why you can get a connection using that.
Unfortunately though USB modems don't work that well with ADSL Max "up to 8 Meg" in the longer term due to the need to disconect/reconnect each time your switch off the PC.
Some routers however work better with longer lines than others. The Speedtouch 585v6 is one. If you don't mind B-grade you can get one from click here for ~£20.
Looking at your stats, if you can manage to maintain a stable conection for 14 days, you should be able to get nearer 2 Meg from your line.
I can't say for certain why your line has suddenly deteriorated but the terrible wet weather has a lot to answer for!