I had exactly the same problem, except I was lucky if it stayed up as long as 20 minutes. I was getting up times of eight minutes or less!
It seems to be OK now, but it took five days to come right.
The registration process gave no warning of this, and the FAQ does tell you you should have a stable connection after an hour. All it says is that the speed will 'fluctuate'. That to my mind does not suggest it keeps dropping the connection. It's made using the internet near impossible for me in the last week.
Anybody using their line for work purposes would be severely stuffed if they upgraded in this way.