Massive loss of sync speed - GFast/Zen Unlimited Fibre 4

Not sure what is going on but my connection has been at a solid 160 meg plus since installation in November. In the last week or so I have noticed a decline in my speed on downloads to the point that I checked my line stats recorded on Zens portal and noted that my sync rate is now sub 100 meg, it has been resyncing every day in the early hours of the morning till it got down to 92 meg.
Any ideas on why this would occur and to such a significant effect? I dare not reboot the openreach modem again as I only seem to lose sync speed after two tries today after speaking with Zen staff, we dropped from 102 sync to 97 and then to 92 after hitting the reset button.

For info - we removed the master socket faceplate and directly attached a ADSL filter - the speed has now doubled from 92 to 185 meg sync rate over the course of 4 days which is much higher than it has ever been

The faceplate what sort was it?

A standard single socket or one with both DSL filter and phone sockets?