A few years ago our speed was terrible.
It was the 3rd engineer who solved it. It was my freeview recorder. It didn't have the capability of connecting to internet or phone, but when he turned it off, his meter registered change in internet signal.
He told us that he once was at a call out where speed was dropping same time every day. Turns out it was caused by kids coming home from school and turning on (I think plasma) TV. Their house was at least a mile from the call out house!!
Could be something like that if occuring same time every day!