Two of the more common reasons for poor connection in wet weather is underground junctions which are not properly sealed, or water getting in to cabinets where the doors don't shut properly.
Can you hear noise on the phone line when you are losing connection?
Try plugging a corded handset in to the master socket (test port if you have one) and try the Quiet Line Test (ring 17070 and chose option 2). If you hear noise on the line, report it to your telephone service provider.
From September 2001 on BTopenworld Home 500/Home 1000/Home 2000. Then ADSLMax on <n>ildram. Moved to ADSL2+ from ADSL24. I'm now with plusnet. I'm not saying who I work for. Any opinions expressed here are my own.