Performance drop 6:30pm - midnight

OK, 

I suffered with this problem for years, but it had gone away until about 7 days ago (Monday last week).

Every evening at pretty much exactly the same time some download speeds drop to a fraction of what they should be.  Youtube seems fine but Twitch but which works great for the day is unusable after 6:30.

Attempts to use ftp also slow to a crawl, my normal speed to a ftp server I use in Holland is 4.4 Mbps, at 6:30 on the dot this drops to about 800 KBs.  If I fire up another connection on my 4G mobile I can see that this is not being caused at the server side as I can still get 4+MBps to the server on 4G even though my Eir connection is giving me 800 kbps.

Speedtest is still showing me my full 40+ Mbs connection, so there is no fault in general with my line, nor is some other device on my network using the bandwidth.

Apart from it looking like eir has once again turned on some form of traffic shaping what could it be?