have you tried connecting to the test socket with a filter and see if that improves your hub stability this will eliminate your extension socket as a problem but you could check that while in test socket there is no dial tone at any of your extension sockets whether in use or not
can you clarify what your mean by 'broadband drops' - internet connection drops or your download speed reduces. if the latter then run btspeedtester and when first test completes then run diagnostic test and post results do test when speed appears ok and again when poor this must be done with wired connection not wireless
which exchange and cab are you on?
The installation of the fibre cabinet will have made no impact on your connection. Can you post the result of your phone number from here (please remember to blank it out before posting): http://dslchecker.bt.com/