Surprise boost in speed off-peak but decrease at other times

I've been enjoying a very reliable, consistent broadband speed of 6 Mbps at all times of day up until recently. But a week ago I was surprised to find I had received a boost of speed to 17mbps. However my delight has been tempered by the fact that at peak times the speed has now become very inconsistent and can now be as low as 2 Mpbs.

Anyone know what might be going on?

Which provider? You may have switched from one wholesale service to another, and the new one has more congestion.

Sorry I should have said it's Namesco.

In which case at a guess you have gone from IPStream Max to WBC ADSL2+ both provided by BT Wholesale

What is your upstream speed, it was probably 448Kbps before, and now is a lot higher.

You need to check the stats from your router, connection speed, attenuation and noise margin, as ADSL2+ is more sensitive to wiring in the home, you may find a need to check at the master socket, and do stuff like remove the ring wire to make it more stable.

If router stats indicate everything is stable, then its probably bad luck that WBC is more congested than the older service on your exchange.