Powerlines can be flaky as they depend on the quality of the electrical circuitry.
If you want the best connection in the loft you should run an ethernet cable from the Hub to there and use that for all the connections.
That will be also be true for all your other connections too - ethernet is always best.
If its problematic then - assuming test ethernet connections are fine - I would suggest getting your own better quality wireless equipment
Can you just "test" on an ethernet connected computer/laptop to see whether your connection is indeed stable - if that is flakey too then you may have a Hub/network connection issue - but that will be revealed if cable connections are dropping.
-------------------- John --------------------
I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.