That sounds much more likely. Since the issue is intermittent, but very much follows an ordinary person's use of the internet (lunch time, after 6 pm, evenings on the weekends). Then all of a sudden it is completely clean for 19 days (vacation?), then it comes back again.
The way of investigating it that you are suggesting is exactly how would go about finding a fault as an audio engineer, but the question is why VM doesn't want to investigate. I have been lied to so much by VM by this point that this has become the worst interaction I have ever had with any service provider. I would never recommend VM to anyone.
Yes, your message was very informative, and I thank you.