Saw exactly the same on the 30th June although didn't get reduced throughput, if it wasn't for the BQM I wouldn't even of noticed it.
I'm not in NI but solved it by turning my router off for 5 mins to get a new IP, it was cured instantly and hasn't happened since.
As you can see it happened 3 evenings running, I forced a new IP on the third evening and the improvement was instant. The large spikes are from the connection being used.
You say it's being happening for four months, I'd be supprised if you've had the same IP for that long so forcing a new one might not work for you in this case, worth a try though.
Note, it's only the router you need to turn off for 5 mins, not the white modem. Good luck.
Edited by Croftie (Fri 24-Aug-12 05:38:56)