Sorry to hear about the drops in speed you are seeing there and I apologise for the inconvenience caused.
Are you seeing these drops in speed over a wired or wireless connection?
I have ran several tests over your broadband and your circuit is in sync at 14.4mbps which is toward the high end of your estimates. No underlying faults have been detected either, that's not to say there isn't. Can you run through our troubleshooting guides here and let us know how you get on?