Hi experts, I have a question about DSL network behaviour. After rebooting my Vigor130 modem it will typically sync to the network at 55M... however several hours later the next time I look at the status screen the speed has reduced to 40M.
Is this normal please? And is there a way to tell the modem to stay connected at the higher speed?

No it isn't normal behaviour and suggests a problem with your line. For the synch speed to reduce your connection must be dropping.

Time to speak to your ISP. Possible causes are a noisy line, can you hear noise/crackling etc. on your phone?

Setting up a TBB quality monitor will help you see how the line is performing :

https://www.thinkbroadband.com/broadband/monitoring/quality