Having been upgraded by Primetel to 8MB several weeks ago, I noticed this morning that my speed had fallen back to 4MB - noticed this on an iPlayer download and then confirmed this via a speedtest.
I was going to call Primetel but before doing so decided to try resetting my modem by switching it off and on again.
Hey Presto - speed back to 8MB.
No idea why - if it happens again maybe I'll call Primetel.