Looks like a problem around 140 MHz, probably noise ingress. The source of the 140 Mhz signal could be a range of things - equipment fault with VM, or assorted permitted private mobile radio use, satellite communication, military and public sector uses. If it's external signals, they can't be stopped, but if VM can find the faulty shielding/ingress point or equipment fault that can be fixed.
A point of noise ingress is usually easily addressable, although if it is intermittent then the field technicians may struggle to observe the fault, and you then might get the "looks good from our side" answer. As VM don't use advanced tools to locate noise faults outcomes are a bit of a mixed bag - many are found and fixed easily, a very, very few are never found and never fixed.
You should check that the connections of the coax cable between hub and wall are properly made and finger tight (ideally undo them and redo them, though that'll knock the hub offline for 5-7 minutes), and if you've got any splitters or TV boxes with unused ports or unused VM coax spurs then check they're fitted with a terminator cap. A reboot, or interrupting the coax may well briefly restore performance because the hub initially re-negotiates a full set of channels, but that won't address any noise ingress issue, so it will then recur as the hub changes frequency "on the fly".
I'll mark your post for the forum staff to advise. If you phone it in, probable outcome is that the offshore script readers will tell you to reboot the hub, it'll initially work fine, and they'll tell you that's fixed, even when the actual problem is not fixed at all.
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Andrew-G many thanks for having a look at it for me.
John_GS thank you too.
I read Andrew-G's post this morning, checked the cables were all tight and tried to do another Samknows/Realspeed test.
However it was stating that my hub was offline (it wasn't as I had internet connectivity)
So I rebooted the hub and then the speed test worked.
As of now I'm getting 1047mbps to the hub. I reboot the hub once per day or so when I notice connectivity or speed issues. General day to day use for browsing / streaming etc seems fine but if I do a big download from Xbox or PSN stores it really shows.
But again, thank you both for your replies and I'll keep an eye on it.