Sudden speed drop on FTTC (after modem restart)

[#1]

I got my FTTC connection just before Christmas and everything was fine.
Then early yesterday morning we had a powercut everything reconnected I didn't think much more of it.
But then while playing games online I noticed massive lag spikes. I've now just run some speed tests and it's seems my speeds have dropped from ~35Mbps to ~25Mbps.
Does anyone know why?
I've tried restarting the routers and reconnecting but it hasn't helped. In fact once I got a speed of ~5Mbps!!!
Have Plusnet just not got the hardware to deal with FTTC connections? Or is there just 1 magic server that works properly and if you're not lucky enough to get on it then you're screwed?
If so I wish they'd mentioned this before I signed up to a new 18 month contract!

[#2]

Have you run a BT speed test?

[#3]

Yes, although if I'm honest that one wasn't as low as the Plusnet and speedtest.net ones (it was around 32Mbps if memory serves).
Didn't run it again since I've been re-connecting as you had to wait an hour.

[#4]

Ah - that might be Plusnet's Traffic Management getting in the way. 8-9PM is pretty much the peak hour - and "browsing" on both "Extra Fibre" and "Value Fibre" is officially slowed to 20Mbps, although I've found that it seems to be more like 24-25Mbps.
Speedtest.net does seem to get slowed down in this limit, but BT's speedtester is not managed (I don't think the testing traffic even goes through Plusnet's routers at all), so you tend to get proper results 24x7.
The important bit of information that you get from running a BT speedtest is not just the test result speeds, but also the "IP Profile" - which tells us how fast your line is sync'ed at. If the profile has lowered, then something is going on with your line. If that has stayed high but the test speeds have dropped, then you are likely to be suffering from the traffic management - and tests should be repeated before 6PM or after 11PM.
Can you post the IP profile from the BT speedtest?
As for the lag spikes - this might just be an unbalanced gateway with some heavy load. Disconnecting (via the router's GUI) and re-connecting can help there, by triggering a switch of gateway. I've found it can make *some* difference, but I don't think I've come across a tremendous difference in behaviour of any of them. Normally the set of pipes works well whatever you connect to.

[#5]

Other people have noticed some problems on both the 3rd and the 4th over on this thread. You might also have been suffering from that as well as the traffic management.

[#6]

I think it was the traffic management stuff.
Tried it again after 11pm and it was back to normal.
Thanks for your help and sorry for the mini rant