I've had this periodically and extremely frustrating it is
I don't think it's anything here and it does go away after a while, I wondered if it was an exchange fault. Certainly when it was happening I had nothing but the router connected, no phones, no filter, and yet it still dropped out. It is practically impossible to get the ISPs to log it because they always assume its you, your filters, or your router. I was trying to track it down to the weather since my entire line is underground and may have been rain ingress to a cabinet however it didn't seem to co-incide.
Thankfully it's not currently happening, but I used to dread the phone call that did it (especially when it was PPI or something) because it put you in that creeping rise in SNR margin that decreased the speed. As I'm 5.7km from the exchange I treasure the 1.7 meg I get and each 3dB takes a lot off that
