I'd go with an LLU provider then, if that's available to you.
Your IP profile on wholesale gets capped to the lowest speed seen in a certain period, so I'm guessing there's occasional bursts of noise that dip your line down to ~800k. It then stays there till reset (or a really long period passes without noise) Most LLU providers don't do this, and give you whatever speed you sync with at the time so a simple reboot fixes it.
I know on Be/O2 LLU I used to get daily noise bursts that dipped my line from 16mb down to about 6mb, a reboot later and I'd be back up to 16 - while my neighbours on BT were permanently stuck on 6

. Be are also offering SRA in some places too IIRC.