Bish knows what he's on about and what he says is true. Locating the exact cause of any issue is trial and error though. All these things being discussed are possibilities. It needs a systematic troubleshooting process to pinpoint, which is why I advocate giving KCOM a call.
When I worked there, people used to get a bit irritated sometimes about being asked to check various different things, but it's the only way to find out for definite what's wrong and fix it. It's very rare that it's just something at the provider's end to fix.