Intermittent service after multiple engineer visits

I'm writing here as I feel I have exhausted any possible avenues through customer services over the past 6 months or so, and I'm currently stuck in an endless loop. After my most recent call today I have a logged yet another complaint, but it seems that the only resolution suggested is to send a third engineer from the same team. For context, I'm running my Hub 5 in modem mode with a third party router, but I've confirmed the issue is still present when using the Hub 5 as a router.

To summarise the current situation:

• At random intervals (this can happen multiple times in a day, or not at all for days) my connection will drop. This typically shows as T3 timeouts (often on all consecutive channels) in the network logs. There is no pattern that I have been able to ascertain.
• The connection will resume after a minute or so without any action needed, but obviously the the loss of connection will impact on anything I am currently doing (work calls, typically).
• The quality of my service other than this is fine, and I have no issues with speed or network quality outside of this particular issue.

I can't put my finger on when the issue first started occurring, but there have been no changes to equipment or in the house since the connection was originally installed. After doing the usual troubleshooting and confirming there were no apparent issues on my end, the first engineer was sent out. The first engineer rebalanced the signals, as some of the power levels were out of spec. A check was also done on all of the equipment internally and externally. The problem still occurred despite this.

After waiting a month or so, I escalated again and a second engineer was sent. The second engineer confirmed the quality of all of the cables and connections, and that there were no apparent issues with my equipment according to their tests. They did check the cabinet and informed me that one of the connections in there was loose (not even finger tight) and that they had rectified that. Unfortunately, the problem has persisted.

Ahead of posting this, I set up Broadband Quality Monitoring and waited for the issue to reoccur. This morning it has happened twice, which I believe is reflected there. I've attached images from today of an example of the issue in the logs, as well as the information from the router, the BQM and the results from SamKnows. Removed all IPs and MAC addresses in the images.

Any help would be greatly appreciated as customer services have essentially told me there is no issue that they can see.