One of my upstream channels is constantly fluctuating between 16 32 and 64 QAM, I’ve had an engineer out and he couldn’t find a problem, he checked my connection in the house and at the cabinet, he also replaced my hub, but this hasn’t solved the problem, the power levels are all around 45db and seem stable,
Any number of factors can cause noise on upstream channels. Faulty amps & non-standard DIY customer wiring to name a few. You cannot see the upstream SNR on VM Hubs (although VM techs can). When I had this problem earlier this year it took several weeks for VM to find the culprit.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.