I dont have the answer but ours does the same. It seems to work fine for most of the morning but starts dropping in the afternoon and gets more frequent in the evening. I thought it was my new laptop and had the USB ports checked by higrade, but this is not the problem.
BT also have done a line check.
I wondered if time played a part, when the exchange bandwidth got overloaded (ie afternoons and evenings) whether a human or "auto" hand killed connections to reduce the load...