Tier 2 should be able to do a direct performance test from your modem to their neighborhood hub.
No such thing on any HFC network that I know of.
Head end --> CMTS --> Node --> User
The node is where the fiber becomes RF/coax. Each node can handle hundreds (thousands?) of customers. If new neighborhoods and new apartment complexes are built and a node has too many users on it, they split the node into two. If there are too many nodes on a CMTS, they move some nodes to another CMTS.
My ISP has 3 CMTSes and I'm not sure how many nodes (probably about 100). It's a local cableco that exists only in this county. The only speed test server they've set up is one on the edge of the network, just before heir own bandwidth provider. It's registered with Ookla, so all my devices always find that one when it responds to ping faster than the others. Testing to the head end should tell if there are any performance bottlenecks along the way. There have been no area performance issues since the network was upgraded in preparation for the DOCSIS 3 upgrade many years ago.
I would expect the bigger operations (Charter, Comcast, etc) to have more than enough capacity. Especially switched digital cable systems where there's a ridiculous amount of bandwidth available for load-balancing cable modems. My first assumption would be: Intermittent noise/interference in the line. That could be due to bad coaxial connections within your home, or even noise feeding into the system from a neighbor. Perhaps your signal quality is borderline and the intermittent noise is just barely enough to push it past that. Wouldn't hurt to have someone make sure the signal is optimal, even if the problem isn't happening when the technician is out there.