No, I think the signal level is too low coming from the street box, but by removing the splitter, the signal reaching the modem is now sufficient. If I plug the splitter and TiVo box back in, I can see the numbers returning to their previous poor levels. I think the signal level when everything is connected is out of spec, but marginal, so it works a lot of the time, but changes in temperature / weather push it over the edge.
The cable from the street is split three ways, serving a TiVo V6 on the ground floor, a TiVo V6 on the first floor, and then finally going to the third floor, where it splits again to feed the cable modem and an original TiVo box. Thus the cable modem is the furthest away from the point of entry, has the longest cable run, and goes through two splitters.
This has worked fairly reliably for 16 years, so I suspect that I’ve been moved onto a different tap point at some stage, or the cable has deteriorated over time. The cabinet is just outside my house, so I can imagine an engineer visiting would assume I need the lowest level tap point, whereas the cable layout is such that there is more cable up the side of my house than from the cabinet to the house.