I get the unfortunate experience of faulting broadband for openreach.
It sounds like it could be what's called a 'hot vp' which, as described by a previous poster is where the virtual port you are connected to is reaching it's maximum capacity. more common in rural exchange areas. keep at them to get an engineer out.
The PO aren't entirely wrong in what they're saying though, as you can get electrical interference from cheaper transformers and stuff that are gradually breaking down. It can be anything and is usually identified by a regular pattern, for example, every time the heating comes on/outside light comes on/etc. my broadband goes off. again really hard to diagnose correctly as it could be anywhere in the local area. (we're not really meant to explain this stuff outside of work but hey ho!)
really difficult to diagnose broadband connection issues remotely as it really could be anything between you and the exchange. my 2p worth if it helps.